Merge branch 'js/diff-notice-has-drive-prefix' into maint

Under certain circumstances, "git diff D:/a/b/c D:/a/b/d" on
Windows would strip initial parts from the paths because they
were not recognized as absolute, which has been corrected.

* js/diff-notice-has-drive-prefix:
  diff: don't attempt to strip prefix from absolute Windows paths

diff.c

@@ -4256,12 +4256,12 @@ static void diff_fill_oid_info(struct diff_filespec *one)
 static void strip_prefix(int prefix_length, const char **namep, const char **otherp)
 {
 	/* Strip the prefix but do not molest /dev/null and absolute paths */
-	if (*namep && **namep != '/') {
+	if (*namep && !is_absolute_path(*namep)) {
 		*namep += prefix_length;
 		if (**namep == '/')
 			++*namep;
 	}
-	if (*otherp && **otherp != '/') {
+	if (*otherp && !is_absolute_path(*otherp)) {
 		*otherp += prefix_length;
 		if (**otherp == '/')
 			++*otherp;

t/t4053-diff-no-index.sh

@@ -127,4 +127,14 @@ test_expect_success 'diff --no-index from repo subdir respects config (implicit)
 	test_cmp expect actual.head
 '
 
+test_expect_success 'diff --no-index from repo subdir with absolute paths' '
+	cat <<-EOF >expect &&
+	1	1	$(pwd)/non/git/{a => b}
+	EOF
+	test_expect_code 1 \
+		git -C repo/sub diff --numstat \
+		"$(pwd)/non/git/a" "$(pwd)/non/git/b" >actual &&
+	test_cmp expect actual
+'
+
 test_done