When "update-index --unresolve $path" cannot find the resolve-undo
record for the path the user requested to unresolve, it stuffs the
blobs from HEAD and MERGE_HEAD to stage #2 and stage #3 as a
fallback. For this reason, the operation does not even start unless
both "HEAD" and "MERGE_HEAD" exist.
This is suboptimal in a few ways:
* It does not recreate stage #1. Even though it is a correct
design decision not to do so (because it is impossible to
recreate in general cases, without knowing how we got there,
including what merge strategy was used), it is much less useful
not to have that information in the index.
* It limits the "unresolve" operation only during a conflicted "git
merge" and nothing else. Other operations like "rebase",
"cherry-pick", and "switch -m" may result in conflicts, and the
user may want to unresolve the conflict that they incorrectly
resolved in order to redo the resolution, but the fallback would
not kick in.
* Most importantly, the entire "unresolve" operation is disabled
after a conflicted merge is committed and MERGE_HEAD is removed,
even though the index has perfectly usable resolve-undo records.
By lazily reading the HEAD and MERGE_HEAD only when we need to go to
the fallback codepath, we will allow cases where resolve-undo
records are available (which is 100% of the time, unless the user is
reading from an index file created by Git more than 10 years ago) to
proceed even after a conflicted merge was committed, during other
mergy operations that do not use MERGE_HEAD, or after the result of
such mergy operations has been committed.
Signed-off-by: Junio C Hamano <gitster@pobox.com>
91e07058c5