quote_path: fix collapsing of relative paths

The code tries to collapse identical leading components
between the prefix and the path. So if we're in "dir1", the
path "dir1/file" should become just "file". However, we were
ending up with "../dir1/file". The included test expected
the wrong output.

The "len" parameter to quote_path can be negative to mean
"this is a NUL terminated string".  Simply count it so that
the loop can rely on it being the length of the path.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t7502-status.sh

@@ -68,7 +68,7 @@ cat > expect << \EOF
 # Changed but not updated:
 #   (use "git add <file>..." to update what will be committed)
 #
-#	modified:   ../dir1/modified
+#	modified:   modified
 #
 # Untracked files:
 #   (use "git add <file>..." to include in what will be committed)

wt-status.c

@@ -82,12 +82,13 @@ static void wt_status_print_trailer(struct wt_status *s)
 }
 
 static char *quote_path(const char *in, int len,
-		struct strbuf *out, const char *prefix)
+			struct strbuf *out, const char *prefix)
 {
-	if (len > 0)
-		strbuf_grow(out, len);
-	strbuf_setlen(out, 0);
+	if (len < 0)
+		len = strlen(in);
 
+	strbuf_grow(out, len);
+	strbuf_setlen(out, 0);
 	if (prefix) {
 		int off = 0;
 		while (prefix[off] && off < len && prefix[off] == in[off])
@@ -104,7 +105,7 @@ static char *quote_path(const char *in, int len,
 				strbuf_addstr(out, "../");
 	}
 
-	for (; (len < 0 && *in) || len > 0; in++, len--) {
+	for ( ; len > 0; in++, len--) {
 		int ch = *in;
 
 		switch (ch) {