imap-send: use skip_prefix instead of using magic numbers

Signed-off-by: Tanay Abhra <tanayabh@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

imap-send.c

@@ -1328,13 +1328,9 @@ static char *imap_folder;
 
 static int git_imap_config(const char *key, const char *val, void *cb)
 {
-	char imap_key[] = "imap.";
-
-	if (strncmp(key, imap_key, sizeof imap_key - 1))
+	if (!skip_prefix(key, "imap.", &key))
 		return 0;
 
-	key += sizeof imap_key - 1;
-
 	/* check booleans first, and barf on others */
 	if (!strcmp("sslverify", key))
 		server.ssl_verify = git_config_bool(key, val);