check-ref-format --branch: strip refs/heads/ using skip_prefix

The expansion returned from strbuf_check_branch_ref always starts with
"refs/heads/" by construction, but there is nothing about its name or
advertised API making that obvious.  This command is used to process
human-supplied input from the command line and is usually not the
inner loop, so we can spare some cycles to be more defensive.  Instead
of hard-coding the offset strlen("refs/heads/") to skip, verify that
the expansion actually starts with refs/heads/.

[jn: split out from a larger patch, added explanation]

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/check-ref-format.c

@@ -39,12 +39,14 @@ static char *collapse_slashes(const char *refname)
 static int check_ref_format_branch(const char *arg)
 {
 	struct strbuf sb = STRBUF_INIT;
+	const char *name;
 	int nongit;
 
 	setup_git_directory_gently(&nongit);
-	if (strbuf_check_branch_ref(&sb, arg))
+	if (strbuf_check_branch_ref(&sb, arg) ||
+	    !skip_prefix(sb.buf, "refs/heads/", &name))
 		die("'%s' is not a valid branch name", arg);
-	printf("%s\n", sb.buf + 11);
+	printf("%s\n", name);
 	return 0;
 }