Chris Packham [off-list ref] writes:
$ git merge topic
Auto-merging foo.c
CONFLICT (content): Merge conflict in foo.c
Auto-merging bar.c
CONFLICT (content): bar.c
Auto-merging otherfile1.c
Auto-merging otherfile2.c
Auto-merging otherfile3.c
Resolved 'foo.c' using previous resolution.
Resolved 'bar.c' using previous resolution.
Automatic merge failed; fix conflicts and then commit the result.
$ git mergetool
No files need merging
So rerere has remembered the bad resolution of foo.c.
I would do this:
$ git rerere forget foo.c
$ git checkout -m foo.c
then fix it up and
$ git add foo.c
$ git commit