combine-diff: factor out stat-format mask

There are several conditionals in the combine diff code that check if
we're doing --stat or --numstat output. Since these must all remain in
sync, let's pull them out into a separate bit-mask.

Arguably this could go into diff.h along with the other DIFF_FORMAT
macros, but it's not clear that the definition of "which formats are
stat" is universal (e.g., does --dirstat count? --summary?). So let's
keep this local to combine-diff.c, where the meaning is more clearly
"stat formats that combine-diff supports".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

combine-diff.c

@@ -1321,6 +1321,11 @@ static const char *path_path(void *obj)
 	return path->path;
 }
 
+/*
+ * Diff stat formats which we always compute solely against the first parent.
+ */
+#define STAT_FORMAT_MASK (DIFF_FORMAT_NUMSTAT \
+			  | DIFF_FORMAT_DIFFSTAT)
 
 /* find set of paths that every parent touches */
 static struct combine_diff_path *find_paths_generic(const struct object_id *oid,
@@ -1342,8 +1347,7 @@ static struct combine_diff_path *find_paths_generic(const struct object_id *oid,
 		 * show stat against the first parent even when doing
 		 * combined diff.
 		 */
-		int stat_opt = (output_format &
-				(DIFF_FORMAT_NUMSTAT|DIFF_FORMAT_DIFFSTAT));
+		int stat_opt = output_format & STAT_FORMAT_MASK;
 		if (i == 0 && stat_opt)
 			opt->output_format = stat_opt;
 		else
@@ -1470,8 +1474,7 @@ void diff_tree_combined(const struct object_id *oid,
 		 * show stat against the first parent even
 		 * when doing combined diff.
 		 */
-		stat_opt = (opt->output_format &
-				(DIFF_FORMAT_NUMSTAT|DIFF_FORMAT_DIFFSTAT));
+		stat_opt = opt->output_format & STAT_FORMAT_MASK;
 		if (stat_opt) {
 			diffopts.output_format = stat_opt;
 
@@ -1515,8 +1518,7 @@ void diff_tree_combined(const struct object_id *oid,
 				show_raw_diff(p, num_parent, rev);
 			needsep = 1;
 		}
-		else if (opt->output_format &
-			 (DIFF_FORMAT_NUMSTAT|DIFF_FORMAT_DIFFSTAT))
+		else if (opt->output_format & STAT_FORMAT_MASK)
 			needsep = 1;
 		else if (opt->output_format & DIFF_FORMAT_CALLBACK)
 			handle_combined_callback(opt, paths, num_parent, num_paths);