$ GIT_TRACE=1 git mergetool
10:55:51.522256 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/bin
10:55:51.524267 git.c:749               trace: exec: git-mergetool
10:55:51.524267 run-command.c:654       trace: run_command: git-mergetool
10:55:51.573183 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:51.648152 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:51.650185 git.c:749               trace: exec: git-sh-i18n--envsubst --variables 'usage: $dashless $USAGE'
10:55:51.650185 run-command.c:654       trace: run_command: git-sh-i18n--envsubst --variables 'usage: $dashless $USAGE'
10:55:51.658184 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:51.678152 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:51.680153 git.c:749               trace: exec: git-sh-i18n--envsubst 'usage: $dashless $USAGE'
10:55:51.680153 run-command.c:654       trace: run_command: git-sh-i18n--envsubst 'usage: $dashless $USAGE'
10:55:51.687154 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:51.758727 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:51.781726 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:51.783727 git.c:460               trace: built-in: git config --bool mergetool.prompt
10:55:51.801727 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:51.802726 git.c:460               trace: built-in: git rev-parse --git-dir
10:55:51.831726 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:51.832726 git.c:460               trace: built-in: git rev-parse --git-path objects
10:55:51.908727 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:51.911728 git.c:460               trace: built-in: git config merge.tool
10:55:51.940726 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:51.942727 git.c:460               trace: built-in: git config mergetool.vimdiff.cmd
10:55:51.993728 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:51.995727 git.c:460               trace: built-in: git config --bool mergetool.keepBackup
10:55:52.023727 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:52.025727 git.c:460               trace: built-in: git config --bool mergetool.keepTemporaries
10:55:52.044727 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:52.045726 git.c:460               trace: built-in: git rev-parse --show-prefix
10:55:52.064726 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:52.065726 git.c:460               trace: built-in: git rev-parse --show-toplevel
10:55:52.089728 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:52.091727 git.c:460               trace: built-in: git rerere remaining
10:55:52.092726 run-command.c:654       trace: run_command: git fsmonitor--daemon start
10:55:52.100727 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:52.103727 git.c:460               trace: built-in: git fsmonitor--daemon start
10:55:52.103727 run-command.c:654       trace: run_command: git fsmonitor--daemon run --detach --ipc-threads=8
10:55:52.134727 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:52.135727 git.c:460               trace: built-in: git diff --name-only --diff-filter=U -- 'a file.txt'
Merging:
a file.txt

10:55:52.158726 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:52.161727 git.c:460               trace: built-in: git ls-files -u -- 'a file.txt'
10:55:52.189727 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:52.191726 git.c:460               trace: built-in: git config mergetool.vimdiff.cmd
10:55:52.231726 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:52.233727 git.c:460               trace: built-in: git config --bool mergetool.writeToTemp
10:55:52.343484 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:52.344483 git.c:460               trace: built-in: git rev-parse --show-cdup
10:55:52.410484 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:52.411483 git.c:460               trace: built-in: git rev-parse --show-cdup
10:55:52.474484 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:52.475483 git.c:460               trace: built-in: git rev-parse --show-cdup
10:55:52.508483 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:52.510483 git.c:460               trace: built-in: git config --type=bool mergetool.vimdiff.hideResolved
10:55:52.529483 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:52.531483 git.c:460               trace: built-in: git config --type=bool mergetool.hideResolved
Normal merge conflict for 'a file.txt':
  {local}: modified file
  {remote}: modified file
10:55:52.567483 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:52.569485 git.c:460               trace: built-in: git config mergetool.vimdiff.cmd
10:55:52.617483 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:52.619483 git.c:460               trace: built-in: git config mergetool.vimdiff.path
10:55:52.653483 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:52.655484 git.c:460               trace: built-in: git config mergetool.vimdiff.cmd
10:55:52.685483 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:52.687483 git.c:460               trace: built-in: git config --bool mergetool.vimdiff.trustExitCode
10:55:52.706534 exec-cmd.c:237          trace: resolved executable dir: C:/Program Files/Git/mingw64/libexec/git-core
10:55:52.708534 git.c:460               trace: built-in: git config mergetool.vimdiff.layout
Vim: Warning: Output is not to a terminal
C:/Program Files/Git/mingw64/libexec/git-core/mergetools/vimdiff: line 417: vertical: command not found
C:/Program Files/Git/mingw64/libexec/git-core/mergetools/vimdiff: line 417: 1b: command not found
C:/Program Files/Git/mingw64/libexec/git-core/mergetools/vimdiff: line 417: wincmd: command not found
C:/Program Files/Git/mingw64/libexec/git-core/mergetools/vimdiff: line 417: vertical: command not found
C:/Program Files/Git/mingw64/libexec/git-core/mergetools/vimdiff: line 417: 2b: command not found
C:/Program Files/Git/mingw64/libexec/git-core/mergetools/vimdiff: line 417: wincmd: command not found
C:/Program Files/Git/mingw64/libexec/git-core/mergetools/vimdiff: line 417: 3b: command not found
C:/Program Files/Git/mingw64/libexec/git-core/mergetools/vimdiff: line 417: wincmd: command not found
C:/Program Files/Git/mingw64/libexec/git-core/mergetools/vimdiff: line 417: 4b: command not found
C:/Program Files/Git/mingw64/libexec/git-core/mergetools/vimdiff: line 417: tabdo: command not found