avoid using skip_prefix as a boolean

There's no point in using:

  if (skip_prefix(buf, "foo"))

over

  if (starts_with(buf, "foo"))

as the point of skip_prefix is to return a pointer to the
data after the prefix. Using starts_with is more readable,
and will make refactoring skip_prefix easier.

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

builtin/fmt-merge-msg.c

@@ -298,7 +298,7 @@ static void credit_people(struct strbuf *out,
 	    (them->nr == 1 &&
 	     me &&
 	     (me = skip_prefix(me, them->items->string)) != NULL &&
-	     skip_prefix(me, " <")))
+	     starts_with(me, " <")))
 		return;
 	strbuf_addf(out, "\n%c %s ", comment_line_char, label);
 	add_people_count(out, them);