compat/mingw: rename the symlink, not the target

[EliahKagan]

This fixes #5436. I've tried to write the commit message (6a994b2) in such a way that it would be suitable both here and upstream, but I do not know if I succeeded at that. As noted in the commit message, and in the issue, existing tests are able to detect the bug this is fixing, so at least for now I have not written any tests.

The code this modifies is also present upstream, but my understanding of the contribution guidelines is that I should open this PR here rather than submitting a patch to the Git mailing list, because the effect of this change is entirely Windows-specific. If this instead needs to be submitted to the mailing list, then I could try and do that. The change here is also rather simple, and I would certainly have no objection to it being integrated in some other way that doesn't involve merging this in any form, if that is more convenient.

[dscho]

Wow, what an excellent Pull Request! Thank you so much!

[dscho]

I verified manually that this fixes it by running:

cd t && MSYS=winsymlinks:nativestrict sh t7001-mv.sh -iVx

With the version from current main (which is identical to v2.48.1.windows.1), it fails with:

[...]
not ok 38 - git mv should overwrite file with a symlink"
#
#               rm -fr .git &&
#               git init &&
#               echo 1 >moved &&
#               test_ln_s_add moved symlink &&
#               git add moved &&
#               test_must_fail git mv symlink moved &&
#               git mv -f symlink moved &&
#               test_path_is_missing symlink &&
#               git update-index --refresh &&
#               git diff-files --quiet
#
1..38

With this patch, it succeeds:

[...]
ok 38 - git mv should overwrite file with a symlink
ok 39 - check moved symlink
ok 40 - setup submodule
ok 41 - git mv cannot move a submodule in a file
ok 42 - git mv moves a submodule with a .git directory and no .gitmodules
ok 43 - git mv moves a submodule with a .git directory and .gitmodules
ok 44 - git mv moves a submodule with gitfile
ok 45 - mv does not complain when no .gitmodules file is found
ok 46 - mv will error out on a modified .gitmodules file unless staged
ok 47 - mv issues a warning when section is not found in .gitmodules
ok 48 - mv --dry-run does not touch the submodule or .gitmodules
ok 49 - checking out a commit before submodule moved needs manual updates
ok 50 - mv -k does not accidentally destroy submodules
ok 51 - moving a submodule in nested directories
ok 52 - moving nested submodules
not ok 53 - nonsense mv triggers assertion failure and partially updated index # TODO known breakage
# still have 1 known breakage(s)
# passed all remaining 52 test(s)
1..53

[dscho]

/add relnote bug A change in upstream Git v2.48.0 broke renaming symlinks, which was fixed.

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