cli: add gvimdiff merge tool, fix docs for diff-invocation-mode

The tool defaults to file-by-file diffing.

Fixes jj-vcs#6206.

Author: ilyagr

cli/src/config/merge_tools.toml

@@ -61,6 +61,25 @@ edit-args = [
     'echomsg "Warning: using plain `vimdiff` as a diff editor is not recommended. See https://github.com/jj-vcs/jj/wiki/Vim for alternatives."',
     "-c", "echohl None",
 ]
+# Not configuring diff-args, as they currently don't seem to work (see
+# https://github.com/jj-vcs/jj/issues/6206)
+
+[merge-tools.gvimdiff]
+program = "gvim"
+# `-d` enables diff mode. `-f` makes vim run in foreground even if it starts a GUI.
+# The other options make sure that only the output file can be modified.
+merge-args = ["-f", "-d", "$output", "-M", "$left", "$base", "$right",
+              "-c", "wincmd J", "-c", "set modifiable", "-c", "set write",
+              "-c", "/<<<<<</+2"]
+merge-tool-edits-conflict-markers = true
+# Directory diffing is barely usable in vimdiff without additional plugins
+edit-args = [
+    "-f", "-d", "$left", "$right", "-c", "echohl WarningMsg", "-c",
+    'echomsg "Warning: using plain `vimdiff` as a diff editor is not recommended. See https://github.com/jj-vcs/jj/wiki/Vim for alternatives."',
+    "-c", "echohl None",
+]
+diff-invocation-mode="file-by-file"
+diff-args = ["-f", "-d", "$left", "$right"]
 
 # if you change the settings for vscode, please do the same for vscodium
 [merge-tools.vscode]

docs/config.md

@@ -283,16 +283,18 @@ diff-args = ["--color=always", "$left", "$right"]
 - `$left` and `$right` are replaced with the paths to the left and right
   directories to diff respectively.
 
-By default `jj` will invoke external tools with a directory containing the left
-and right sides.  The `diff-invocation-mode` config can change this to file by file
-invocations as follows:
+By default `jj` will invoke most external tools with a directory containing the left
+and right sides.
+
+For diff tools, the `diff-invocation-mode` config can be used to toggle  between
+this behavior and file-by-file invocations:
 
 ```toml
 [ui]
-diff.tool = "vimdiff"
+diff.tool = "gvimdiff"
 
 [merge-tools.vimdiff]
-diff-invocation-mode = "file-by-file"
+diff-invocation-mode = "dir"  # Or "file-by-file"
 ```
 
 By default `jj` will display a warning when the command exits with a non-success