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>

pretty.c

@@ -40,10 +40,9 @@ static int git_pretty_formats_config(const char *var, const char *value, void *c
 	const char *fmt;
 	int i;
 
-	if (!starts_with(var, "pretty."))
+	if (!skip_prefix(var, "pretty.", &name))
 		return 0;
 
-	name = var + strlen("pretty.");
 	for (i = 0; i < builtin_formats_len; i++) {
 		if (!strcmp(commit_formats[i].name, name))
 			return 0;