use skip_prefix to avoid repeated calculations

In some cases, we use starts_with to check for a prefix, and
then use an already-calculated prefix length to advance a
pointer past the prefix. There are no magic numbers or
duplicated strings here, but we can still make the code
simpler and more obvious by using skip_prefix.

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

help.c

@@ -129,7 +129,6 @@ static void list_commands_in_dir(struct cmdnames *cmds,
 					 const char *path,
 					 const char *prefix)
 {
-	int prefix_len;
 	DIR *dir = opendir(path);
 	struct dirent *de;
 	struct strbuf buf = STRBUF_INIT;
@@ -139,27 +138,27 @@ static void list_commands_in_dir(struct cmdnames *cmds,
 		return;
 	if (!prefix)
 		prefix = "git-";
-	prefix_len = strlen(prefix);
 
 	strbuf_addf(&buf, "%s/", path);
 	len = buf.len;
 
 	while ((de = readdir(dir)) != NULL) {
+		const char *ent;
 		int entlen;
 
-		if (!starts_with(de->d_name, prefix))
+		if (!skip_prefix(de->d_name, prefix, &ent))
 			continue;
 
 		strbuf_setlen(&buf, len);
 		strbuf_addstr(&buf, de->d_name);
 		if (!is_executable(buf.buf))
 			continue;
 
-		entlen = strlen(de->d_name) - prefix_len;
-		if (has_extension(de->d_name, ".exe"))
+		entlen = strlen(ent);
+		if (has_extension(ent, ".exe"))
 			entlen -= 4;
 
-		add_cmdname(cmds, de->d_name + prefix_len, entlen);
+		add_cmdname(cmds, ent, entlen);
 	}
 	closedir(dir);
 	strbuf_release(&buf);

http.c

@@ -1087,11 +1087,10 @@ static int update_url_from_redirect(struct strbuf *base,
 	if (!strcmp(asked, got->buf))
 		return 0;
 
-	if (!starts_with(asked, base->buf))
+	if (!skip_prefix(asked, base->buf, &tail))
 		die("BUG: update_url_from_redirect: %s is not a superset of %s",
 		    asked, base->buf);
 
-	tail = asked + base->len;
 	tail_len = strlen(tail);
 
 	if (got->len < tail_len ||