xdl_change_compact(): only use heuristic if group can't be matched

If the changed group of lines can be matched to a group in the other
file, then that positioning should take precedence over the compaction
heuristic.

The old code tried the heuristic unconditionally, which cost redundant
effort and also was broken if the matching code had already shifted the
group higher than the blank line.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

xdiff/xdiffi.c

@@ -504,25 +504,25 @@ int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
 			}
 		} while (grpsiz != ix - ixs);
 
-		/*
-		 * Try to move back the possibly merged group of changes, to match
-		 * the recorded position in the other file.
-		 */
-		while (ixref < ix) {
-			rchg[--ixs] = 1;
-			rchg[--ix] = 0;
-			while (rchgo[--ixo]);
-		}
-
-		/*
-		 * If a group can be moved back and forth, see if there is a
-		 * blank line in the moving space. If there is a blank line,
-		 * make sure the last blank line is the end of the group.
-		 *
-		 * As we already shifted the group forward as far as possible
-		 * in the earlier loop, we need to shift it back only if at all.
-		 */
-		if ((flags & XDF_COMPACTION_HEURISTIC) && blank_lines) {
+		if (ixref < ix) {
+			/*
+			 * Try to move back the possibly merged group of changes, to match
+			 * the recorded position in the other file.
+			 */
+			while (ixref < ix) {
+				rchg[--ixs] = 1;
+				rchg[--ix] = 0;
+				while (rchgo[--ixo]);
+			}
+		} else if ((flags & XDF_COMPACTION_HEURISTIC) && blank_lines) {
+			/*
+			 * The group can be slid up to make its last line a
+			 * blank line. Do so.
+			 *
+			 * As we already shifted the group forward as far as
+			 * possible in the earlier loop, we need to shift it
+			 * back only if at all.
+			 */
 			while (ixs > 0 &&
 			       !is_blank_line(recs, ix - 1, flags) &&
 			       recs_match(recs, ixs - 1, ix - 1, flags)) {