C: use skip_prefix() to avoid hardcoded string length

We often skip an optional prefix in a string with a hardcoded
constant, e.g.

	if (starts_with(string, "prefix"))
		string += 6;

which is less error prone when written

	skip_prefix(string, "prefix", &string);

Note that this changes a few error messages from "git reflog expire
--expire=nonsense.timestamp", which used to complain by saying

    '--expire=nonsense.timestamp' is not a valid timestamp

but with this change, we say

    'nonsense.timestamp' is not a valid timestamp

which is more technically correct (the string with --expire= as
a prefix obviously cannot be a valid timestamp, but the error is
about the part of the input without that prefix).

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 1 insertions, 2 deletions

diff --git a/refs/files-backend.c b/refs/files-backend.c
index 0ea66a28b6..561c33ac8a 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -465,8 +465,7 @@ stat_ref:
 	close(fd);
 	strbuf_rtrim(&sb_contents);
 	buf = sb_contents.buf;
-	if (starts_with(buf, "ref:")) {
-		buf += 4;
+	if (skip_prefix(buf, "ref:", &buf)) {
 		while (isspace(*buf))
 			buf++;