merge_trees(): add ancestor label parameter for diff3-style output

Commands using the merge_trees() machinery will present conflict hunks
in output something like what ‘diff3 -m’ produces if the
merge.conflictstyle configuration option is set to diff3.  The output
lacks the name of the merge base on the ||||||| line of the output,
and tools can misparse the conflict hunks without it.  Add a new
o->ancestor parameter to merge_trees() for use as a label for the
ancestor in conflict hunks.

If o->ancestor is NULL, the output format is as before.  All callers
pass NULL for now.

If o->ancestor is non-NULL and both branches renamed the base file
to the same name, that name is included in the conflict hunk labels.
Even if o->ancestor is NULL I think this would be a good change, but
this patch only does it in the non-NULL case to ensure the output
format does not change where it might matter.

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>

merge-recursive.c

@@ -608,7 +608,7 @@ static int merge_3way(struct merge_options *o,
 		      const char *branch2)
 {
 	mmfile_t orig, src1, src2;
-	char *name1, *name2;
+	char *base_name, *name1, *name2;
 	int merge_status;
 	int favor;
 
@@ -628,10 +628,15 @@ static int merge_3way(struct merge_options *o,
 		}
 	}
 
-	if (strcmp(a->path, b->path)) {
+	if (strcmp(a->path, b->path) ||
+	    (o->ancestor != NULL && strcmp(a->path, one->path) != 0)) {
+		base_name = o->ancestor == NULL ? NULL :
+			xstrdup(mkpath("%s:%s", o->ancestor, one->path));
 		name1 = xstrdup(mkpath("%s:%s", branch1, a->path));
 		name2 = xstrdup(mkpath("%s:%s", branch2, b->path));
 	} else {
+		base_name = o->ancestor == NULL ? NULL :
+			xstrdup(mkpath("%s", o->ancestor));
 		name1 = xstrdup(mkpath("%s", branch1));
 		name2 = xstrdup(mkpath("%s", branch2));
 	}
@@ -640,7 +645,7 @@ static int merge_3way(struct merge_options *o,
 	read_mmblob(&src1, a->sha1);
 	read_mmblob(&src2, b->sha1);
 
-	merge_status = ll_merge(result_buf, a->path, &orig, NULL,
+	merge_status = ll_merge(result_buf, a->path, &orig, base_name,
 				&src1, name1, &src2, name2,
 				(!!o->call_depth) | (favor << 1));
 

merge-recursive.h

@@ -4,6 +4,7 @@
 #include "string-list.h"
 
 struct merge_options {
+	const char *ancestor;
 	const char *branch1;
 	const char *branch2;
 	enum {