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>

builtin/merge-file.c

@@ -77,6 +77,7 @@ int cmd_merge_file(int argc, const char **argv, const char *prefix)
 					argv[i]);
 	}
 
+	xmp.ancestor = names[1];
 	xmp.file1 = names[0];
 	xmp.file2 = names[2];
 	ret = xdl_merge(mmfs + 1, mmfs + 0, mmfs + 2, &xmp, &result);

t/t6023-merge-file.sh

@@ -181,7 +181,7 @@ et nihil mihi deerit;
 
 In loco pascuae ibi me collocavit;
 super aquam refectionis educavit me.
-|||||||
+||||||| new5.txt
 et nihil mihi deerit.
 In loco pascuae ibi me collocavit,
 super aquam refectionis educavit me;
@@ -225,7 +225,7 @@ et nihil mihi deerit;
 
 In loco pascuae ibi me collocavit;
 super aquam refectionis educavit me.
-||||||||||
+|||||||||| new5.txt
 et nihil mihi deerit.
 In loco pascuae ibi me collocavit,
 super aquam refectionis educavit me;