xdiff: give up scanning similar lines early

In a corner case of large files whose lines do not match uniquely, the
loop to eliminate a line that matches multiple locations adjacent to a run
of lines that do not uniquely match wasted too much cycles.  Fix this by
giving up early after scanning 100 lines in both direction.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

xdiff/xprepare.c

@@ -23,10 +23,9 @@
 #include "xinclude.h"
 
 
-
 #define XDL_KPDIS_RUN 4
 #define XDL_MAX_EQLIMIT 1024
-
+#define XDL_SIMSCAN_WINDOW 100
 
 
 typedef struct s_xdlclass {
@@ -312,6 +311,18 @@ void xdl_free_env(xdfenv_t *xe) {
 static int xdl_clean_mmatch(char const *dis, long i, long s, long e) {
 	long r, rdis0, rpdis0, rdis1, rpdis1;
 
+	/*
+	 * Limits the window the is examined during the similar-lines
+	 * scan. The loops below stops when dis[i - r] == 1 (line that
+	 * has no match), but there are corner cases where the loop
+	 * proceed all the way to the extremities by causing huge
+	 * performance penalties in case of big files.
+	 */
+	if (i - s > XDL_SIMSCAN_WINDOW)
+		s = i - XDL_SIMSCAN_WINDOW;
+	if (e - i > XDL_SIMSCAN_WINDOW)
+		e = i + XDL_SIMSCAN_WINDOW;
+
 	/*
 	 * Scans the lines before 'i' to find a run of lines that either
 	 * have no match (dis[j] == 0) or have multiple matches (dis[j] > 1).