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>