resolve-undo: teach "update-index --unresolve" to use resolve-undo info

The update-index plumbing command had a hacky --unresolve implementation
that was written back in the days when merge was the only way for users to
end up with higher stages in the index, and assumed that stage #2 must
have come from HEAD, stage #3 from MERGE_HEAD and didn't bother to compute
the stage #1 information.

There were several issues with this approach:

 - These days, merge is not the only command, and conflicts coming from
   commands like cherry-pick, "am -3", etc. cannot be recreated by looking
   at MERGE_HEAD;

 - For a conflict that came from a merge that had renames, picking up the
   same path from MERGE_HEAD and HEAD wouldn't help recreating it, either;

 - It may have been Ok not to recreate stage #1 back when it was written,
   because "diff --ours/--theirs" were the only availble ways to review
   conflicts and they don't need stage #1 information.  "diff --cc" that
   was invented much later is a lot more useful way but it needs stage #1.

We can use resolve-undo information recorded in the index extension to
solve all of these issues.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/t/t2030-unresolve-info.sh b/t/t2030-unresolve-info.sh
index ea65f391c1..28e2eb1cec 100755
--- a/t/t2030-unresolve-info.sh
+++ b/t/t2030-unresolve-info.sh
@@ -108,4 +108,11 @@ test_expect_success 'add records checkout -m undoes' '
 	grep "^++<<<<<<<" actual
 '
 
+test_expect_success 'unmerge with plumbing' '
+	prime_resolve_undo &&
+	git update-index --unresolve file &&
+	git ls-files -u >actual &&
+	test $(wc -l <actual) = 3
+'
+
 test_done