use skip_prefix to avoid repeating strings

It's a common idiom to match a prefix and then skip past it
with strlen, like:

  if (starts_with(foo, "bar"))
	  foo += strlen("bar");

This avoids magic numbers, but means we have to repeat the
string (and there is no compiler check that we didn't make a
typo in one of the strings).

We can use skip_prefix to handle this case without repeating
ourselves.

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

sha1_name.c

@@ -911,10 +911,8 @@ static int grab_nth_branch_switch(unsigned char *osha1, unsigned char *nsha1,
 	const char *match = NULL, *target = NULL;
 	size_t len;
 
-	if (starts_with(message, "checkout: moving from ")) {
-		match = message + strlen("checkout: moving from ");
+	if (skip_prefix(message, "checkout: moving from ", &match))
 		target = strstr(match, " to ");
-	}
 
 	if (!match || !target)
 		return 0;