diff: fix a double off-by-one with --ignore-space-at-eol

When comparing two lines, ignoring any whitespace at the end, we first
try to match as many bytes as possible and break out of the loop only
upon mismatch, to let the remainder be handled by the code shared with
the other whitespace-ignoring code paths.

When comparing the bytes, however, we incremented the counters always,
even if the bytes did not match. And because we fall through to  the
space-at-eol handling at that point, it is as if that mismatch never
happened.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4033-diff-patience.sh

@@ -5,7 +5,7 @@ test_description='patience diff algorithm'
 . ./test-lib.sh
 . "$TEST_DIRECTORY"/lib-diff-alternative.sh
 
-test_expect_failure '--ignore-space-at-eol with a single appended character' '
+test_expect_success '--ignore-space-at-eol with a single appended character' '
 	printf "a\nb\nc\n" >pre &&
 	printf "a\nbX\nc\n" >post &&
 	test_must_fail git diff --no-index \

xdiff/xpatience.c

@@ -1,6 +1,6 @@
 /*
  *  LibXDiff by Davide Libenzi ( File Differential Library )
- *  Copyright (C) 2003-2009 Davide Libenzi, Johannes E. Schindelin
+ *  Copyright (C) 2003-2016 Davide Libenzi, Johannes E. Schindelin
  *
  *  This library is free software; you can redistribute it and/or
  *  modify it under the terms of the GNU Lesser General Public

xdiff/xutils.c

@@ -200,8 +200,10 @@ int xdl_recmatch(const char *l1, long s1, const char *l2, long s2, long flags)
 				return 0;
 		}
 	} else if (flags & XDF_IGNORE_WHITESPACE_AT_EOL) {
-		while (i1 < s1 && i2 < s2 && l1[i1++] == l2[i2++])
-			; /* keep going */
+		while (i1 < s1 && i2 < s2 && l1[i1] == l2[i2]) {
+			i1++;
+			i2++;
+		}
 	}
 
 	/*