merge-file --diff3: add a label for ancestor

merge-file --diff3: add a label for ancestor

git merge-file --diff3 can be used to present conflicts hunks
including text from the common ancestor.

The added information is helpful for resolving a merge by hand, and
merge tools can usually grok it because it looks like output from
diff3 -m.  However, ‘diff3’ includes a label for the merge base on the
||||||| line and some tools cannot parse conflict hunks without such a
label.  Write the base-name as passed in a -L option (or the name of
the ancestor file by default) on that line.

git rerere will not have trouble parsing this output, since instead of
looking for a newline, it looks for whitespace after the |||||||
marker.  Since rerere includes its own code for recreating conflict
hunks, conflict identifiers are unaffected.  No other code in git tries
to parse conflict hunks.

Requested-by: Stefan Monnier <monnier@iro.umontreal.ca>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>