Re: git format-patch lost the last part when branch merge

On Mon, Feb 22, 2021 at 3:03 PM Jeff King [off-list ref] wrote:
[...snip...]

There are lots of ways to look at the diff of a merge. By default, `git
show` will show a combined diff, which omits hunks where one side was
taken verbatim, but otherwise shows what each side did.

The diff shown in the link above is a diff against the first-parent
(which you can also get locally with `git show --first-parent 582cd91`).
One _could_ apply that diff onto the first parent to achieve the same
tree as the merge plus all of the commits that got merged in. But it
wouldn't make any sense to apply that (aside from conflict resolution,
it would be redundant with all of the commits that format-patch just
output!).

You could imagine ways for format-patch to represent the conflict
resolution done in a merge, but it's not quite trivial, and nobody has
done it yet.

Are you referring to something like this (in the git.git repository)?

$ git show --oneline --remerge-diff 42342b3ee6
42342b3ee6 Merge branch 'ab/mailmap'

diff --git a/t/t4203-mailmap.sh b/t/t4203-mailmap.sh
index 1bce961e07..6fb18a34b0 100755
--- a/t/t4203-mailmap.sh
+++ b/t/t4203-mailmap.sh

@@ -257,16 +257,8 @@ test_expect_success 'No mailmap files, but configured' '

 test_expect_success 'setup mailmap blob tests' '
        git checkout -b map &&
-<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< 60ecad090d (Merge branch 'ps/fetch-atomic')
        test_when_finished "git checkout main" &&
-       cat >just-bugs <<- EOF &&
-|||||||||||||||||||||||||||||||| 72c4083ddf
-       test_when_finished "git checkout master" &&
-       cat >just-bugs <<- EOF &&
-================================
-       test_when_finished "git checkout master" &&
        cat >just-bugs <<-\EOF &&
->>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 4e168333a8 (shortlog: remove

unused(?) "repo-abbrev" feature)
        Blob Guy [off-list ref]
        EOF
        cat >both <<-EOF &&


I agree that representing the conflict done in a merge is more
difficult than it sounds, but if you mean what I think you mean then I
disagree with the "nobody has done it yet" half of your statement.
:-)

That said, I haven't attempted to tie this into format-patch in any
way, and have absolutely no plans to.  (And --remerge-diff hasn't been
submitted upstream, because it depends on ort, and that still needs
reviews...)