diff: handle lines containing only whitespace and tabs better

When a line contains nothing but whitespace with at least one tab
and the core.whitespace config option contains blank-at-eol, the
whitespace on the line is being printed twice, once unhighlighted
(unless otherwise matched by one of the other core.whitespace values),
and a second time highlighted for blank-at-eol.

Update the leading indentation check to stop checking when it reaches
the trailing whitespace.

Signed-off-by: Kevin Ballard <kevin@sb.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4015-diff-whitespace.sh

diff --git a/t/t4015-diff-whitespace.sh b/t/t4015-diff-whitespace.sh
index 935d101..a8736f7 100755
--- a/t/t4015-diff-whitespace.sh
+++ b/t/t4015-diff-whitespace.sh
@@ -491,4 +491,41 @@
 
 '
 
+# Start testing the colored format for whitespace checks
+
+test_expect_success 'setup diff colors' '
+	git config color.diff always &&
+	git config color.diff.plain normal &&
+	git config color.diff.meta bold &&
+	git config color.diff.frag cyan &&
+	git config color.diff.func normal &&
+	git config color.diff.old red &&
+	git config color.diff.new green &&
+	git config color.diff.commit yellow &&
+	git config color.diff.whitespace "normal red" &&
+
+	git config core.autocrlf false
+'
+cat >expected <<\EOF
+<BOLD>diff --git a/x b/x<RESET>
+<BOLD>index 9daeafb..2874b91 100644<RESET>
+<BOLD>--- a/x<RESET>
+<BOLD>+++ b/x<RESET>
+<CYAN>@@ -1 +1,4 @@<RESET>
+ test<RESET>
+<GREEN>+<RESET><GREEN>{<RESET>
+<GREEN>+<RESET><BRED>	<RESET>
+<GREEN>+<RESET><GREEN>}<RESET>
+EOF
+
+test_expect_success 'diff that introduces a line with only tabs' '
+	git config core.whitespace blank-at-eol &&
+	git reset --hard &&
+	echo "test" > x &&
+	git commit -m "initial" x &&
+	echo "{NTN}" | tr "NT" "\n\t" >> x &&
+	git -c color.diff=always diff | test_decode_color >current &&
+	test_cmp expected current
+'
+
 test_done

ws.c

diff --git a/ws.c b/ws.c
index d7b8c33..7302f8f 100644
--- a/ws.c
+++ b/ws.c
@@ -174,8 +174,11 @@
 		}
 	}
 
+	if (trailing_whitespace == -1)
+		trailing_whitespace = len;
+
 	/* Check indentation */
-	for (i = 0; i < len; i++) {
+	for (i = 0; i < trailing_whitespace; i++) {
 		if (line[i] == ' ')
 			continue;
 		if (line[i] != '\t')
@@ -218,8 +221,6 @@
 		 * Now the rest of the line starts at "written".
 		 * The non-highlighted part ends at "trailing_whitespace".
 		 */
-		if (trailing_whitespace == -1)
-			trailing_whitespace = len;
 
 		/* Emit non-highlighted (middle) segment. */
 		if (trailing_whitespace - written > 0) {