Start the merging-rebase of 0cf6f62..512e5d6 onto v2.15.1

The commit range 0cf6f62..512e5d6 represents the patches
accumulated by the Git for Windows project since the latest
merging-rebase (including the rebased patches, and all kinds of
fixup!/squash! commits intended to update or drop selected patches).

With this commit, no functional changes are introduced, but the
fixup!/squash! commits are applied as appropriate, and the
`vs2017_vcpkg` branch is rebased onto the current revision of the
`vs2017_vcxproj` branch.

While at it, the latest two commits (touching the README as well as our
version of core Git's Pull Request template) are moved into the `readme`
branch.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho