xdl_merge(): add optional ancestor label to diff3-style output

The ‘git checkout --conflict=diff3’ command can be used to
present conflicts hunks including text from the common ancestor:

	<<<<<<< ours
	ourside
	|||||||
	original
	=======
	theirside
	>>>>>>> theirs

The added information is helpful for resolving merges by hand, and
merge tools can usually grok it because it is very similar to the
output from diff3 -m.

A subtle change can help more tools to understand the output.  ‘diff3’
includes the name of the merge base on the ||||||| line of the output,
and some tools misparse the conflict hunks without it.  Add a new
xmp->ancestor parameter to xdl_merge() for use with conflict style
XDL_MERGE_DIFF3 as a label on the ||||||| line for any conflict hunks.

If xmp->ancestor is NULL, the output format is unchanged.  Thus, this
change only provides unexposed plumbing for the new feature; it does
not affect the outward behavior of git.

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

xdiff/xdiff.h

@@ -117,6 +117,7 @@ typedef struct s_xmparam {
 	int level;
 	int favor;
 	int style;
+	const char *ancestor;	/* label for orig */
 } xmparam_t;
 
 #define DEFAULT_CONFLICT_MARKER_SIZE 7

xdiff/xmerge.c

@@ -145,11 +145,13 @@ static int xdl_orig_copy(xdfenv_t *xe, int i, int count, int add_nl, char *dest)
 
 static int fill_conflict_hunk(xdfenv_t *xe1, const char *name1,
 			      xdfenv_t *xe2, const char *name2,
+			      const char *name3,
 			      int size, int i, int style,
 			      xdmerge_t *m, char *dest, int marker_size)
 {
 	int marker1_size = (name1 ? strlen(name1) + 1 : 0);
 	int marker2_size = (name2 ? strlen(name2) + 1 : 0);
+	int marker3_size = (name3 ? strlen(name3) + 1 : 0);
 	int j;
 
 	if (marker_size <= 0)
@@ -179,10 +181,15 @@ static int fill_conflict_hunk(xdfenv_t *xe1, const char *name1,
 	if (style == XDL_MERGE_DIFF3) {
 		/* Shared preimage */
 		if (!dest) {
-			size += marker_size + 1;
+			size += marker_size + 1 + marker3_size;
 		} else {
 			for (j = 0; j < marker_size; j++)
 				dest[size++] = '|';
+			if (marker3_size) {
+				dest[size] = ' ';
+				memcpy(dest + size + 1, name3, marker3_size - 1);
+				size += marker3_size;
+			}
 			dest[size++] = '\n';
 		}
 		size += xdl_orig_copy(xe1, m->i0, m->chg0, 1,
@@ -217,6 +224,7 @@ static int fill_conflict_hunk(xdfenv_t *xe1, const char *name1,
 
 static int xdl_fill_merge_buffer(xdfenv_t *xe1, const char *name1,
 				 xdfenv_t *xe2, const char *name2,
+				 const char *ancestor_name,
 				 int favor,
 				 xdmerge_t *m, char *dest, int style,
 				 int marker_size)
@@ -229,6 +237,7 @@ static int xdl_fill_merge_buffer(xdfenv_t *xe1, const char *name1,
 
 		if (m->mode == 0)
 			size = fill_conflict_hunk(xe1, name1, xe2, name2,
+						  ancestor_name,
 						  size, i, style, m, dest,
 						  marker_size);
 		else if (m->mode & 3) {
@@ -403,6 +412,7 @@ static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1, const char *name1,
 		xmparam_t const *xmp, mmbuffer_t *result) {
 	xdmerge_t *changes, *c;
 	xpparam_t const *xpp = &xmp->xpp;
+	const char *const ancestor_name = xmp->ancestor;
 	int i0, i1, i2, chg0, chg1, chg2;
 	int level = xmp->level;
 	int style = xmp->style;
@@ -540,6 +550,7 @@ static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1, const char *name1,
 	if (result) {
 		int marker_size = xmp->marker_size;
 		int size = xdl_fill_merge_buffer(xe1, name1, xe2, name2,
+						 ancestor_name,
 						 favor, changes, NULL, style,
 						 marker_size);
 		result->ptr = xdl_malloc(size);
@@ -548,7 +559,8 @@ static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1, const char *name1,
 			return -1;
 		}
 		result->size = size;
-		xdl_fill_merge_buffer(xe1, name1, xe2, name2, favor, changes,
+		xdl_fill_merge_buffer(xe1, name1, xe2, name2,
+				      ancestor_name, favor, changes,
 				      result->ptr, style, marker_size);
 	}
 	return xdl_cleanup_merge(changes);