diff: add a comment about combine_diff_path.parent.path

We only fill in the per-parent "path" field when it differs from what's
in combine_diff_path.path (and even then only when the option is
appropriate). Let's document that.

Suggested-by: Wink Saville <wink@saville.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.h

@@ -480,6 +480,12 @@ struct combine_diff_path {
 		char status;
 		unsigned int mode;
 		struct object_id oid;
+		/*
+		 * This per-parent path is filled only when doing a combined
+		 * diff with revs.combined_all_paths set, and only if the path
+		 * differs from the post-image (e.g., a rename or copy).
+		 * Otherwise it is left NULL.
+		 */
 		char *path;
 	} parent[FLEX_ARRAY];
 };