diff: prevent pprint_rename from underrunning input

The logic described in d020e27 (diff: Fix rename pretty-print when
suffix and prefix overlap, 2013-02-23) is wrong: The proof in the
comment is valid only if both strings are the same length.  *One* of
old/new can reach a-1 (b-1, resp.) if 'a' is a suffix of 'b' (or vice
versa).

Since the intent was to let the loop run down to the '/' at the end of
the common prefix, fix it by making that distinction explicit: if
there is no prefix, allow no underrun.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -1151,6 +1151,7 @@ static char *pprint_rename(const char *a, const char *b)
 	const char *new = b;
 	struct strbuf name = STRBUF_INIT;
 	int pfx_length, sfx_length;
+	int pfx_adjust_for_slash;
 	int len_a = strlen(a);
 	int len_b = strlen(b);
 	int a_midlen, b_midlen;
@@ -1178,14 +1179,16 @@ static char *pprint_rename(const char *a, const char *b)
 	new = b + len_b;
 	sfx_length = 0;
 	/*
-	 * Note:
-	 * if pfx_length is 0, old/new will never reach a - 1 because it
-	 * would mean the whole string is common suffix. But then, the
-	 * whole string would also be a common prefix, and we would not
-	 * have pfx_length equals 0.
+	 * If there is a common prefix, it must end in a slash.  In
+	 * that case we let this loop run 1 into the prefix to see the
+	 * same slash.
+	 *
+	 * If there is no common prefix, we cannot do this as it would
+	 * underrun the input strings.
 	 */
-	while (a + pfx_length - 1 <= old &&
-	       b + pfx_length - 1 <= new &&
+	pfx_adjust_for_slash = (pfx_length ? 1 : 0);
+	while (a + pfx_length - pfx_adjust_for_slash <= old &&
+	       b + pfx_length - pfx_adjust_for_slash <= new &&
 	       *old == *new) {
 		if (*old == '/')
 			sfx_length = len_a - (old - a);