When writing conflict hunks in ‘diff3 -m’ format, also add a label to
the common ancestor.  Especially in a cherry-pick, it is not immediately
obvious without such a label what the common ancestor represents.

git rerere does not have trouble parsing the new output and its preimage
ids are unchanged since it includes its own code for recreating conflict
hunks.  No other code in git parses conflict hunks.
Requested-by: NStefan Monnier <monnier@iro.umontreal.ca>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>