C: use skip_prefix() to avoid hardcoded string length

We often skip an optional prefix in a string with a hardcoded
constant, e.g.

	if (starts_with(string, "prefix"))
		string += 6;

which is less error prone when written

	skip_prefix(string, "prefix", &string);

Note that this changes a few error messages from "git reflog expire
--expire=nonsense.timestamp", which used to complain by saying

    '--expire=nonsense.timestamp' is not a valid timestamp

but with this change, we say

    'nonsense.timestamp' is not a valid timestamp

which is more technically correct (the string with --expire= as
a prefix obviously cannot be a valid timestamp, but the error is
about the part of the input without that prefix).

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

builtin/fast-export.c

@@ -870,8 +870,7 @@ static void handle_tag(const char *name, struct tag *tag)
 		printf("reset %s\nfrom %s\n\n",
 		       name, oid_to_hex(&null_oid));
 	}
-	if (starts_with(name, "refs/tags/"))
-		name += 10;
+	skip_prefix(name, "refs/tags/", &name);
 	printf("tag %s\n", name);
 	if (mark_tags) {
 		mark_next_object(&tag->object);

builtin/reflog.c

@@ -560,15 +560,16 @@ static int cmd_reflog_expire(int argc, const char **argv, const char *prefix)
 
 	for (i = 1; i < argc; i++) {
 		const char *arg = argv[i];
+
 		if (!strcmp(arg, "--dry-run") || !strcmp(arg, "-n"))
 			flags |= EXPIRE_REFLOGS_DRY_RUN;
-		else if (starts_with(arg, "--expire=")) {
-			if (parse_expiry_date(arg + 9, &cb.cmd.expire_total))
+		else if (skip_prefix(arg, "--expire=", &arg)) {
+			if (parse_expiry_date(arg, &cb.cmd.expire_total))
 				die(_("'%s' is not a valid timestamp"), arg);
 			explicit_expiry |= EXPIRE_TOTAL;
 		}
-		else if (starts_with(arg, "--expire-unreachable=")) {
-			if (parse_expiry_date(arg + 21, &cb.cmd.expire_unreachable))
+		else if (skip_prefix(arg, "--expire-unreachable=", &arg)) {
+			if (parse_expiry_date(arg, &cb.cmd.expire_unreachable))
 				die(_("'%s' is not a valid timestamp"), arg);
 			explicit_expiry |= EXPIRE_UNREACH;
 		}

notes.c

@@ -1279,10 +1279,8 @@ static void format_note(struct notes_tree *t, const struct object_id *object_oid
 		if (!ref || !strcmp(ref, GIT_NOTES_DEFAULT_REF)) {
 			strbuf_addstr(sb, "\nNotes:\n");
 		} else {
-			if (starts_with(ref, "refs/"))
-				ref += 5;
-			if (starts_with(ref, "notes/"))
-				ref += 6;
+			skip_prefix(ref, "refs/", &ref);
+			skip_prefix(ref, "notes/", &ref);
 			strbuf_addf(sb, "\nNotes (%s):\n", ref);
 		}
 	}

parse-options.c

@@ -357,8 +357,7 @@ is_abbreviated:
 			}
 			/* negated? */
 			if (!starts_with(arg, "no-")) {
-				if (starts_with(long_name, "no-")) {
-					long_name += 3;
+				if (skip_prefix(long_name, "no-", &long_name)) {
 					opt_flags |= OPT_UNSET;
 					goto again;
 				}

refs/files-backend.c

@@ -465,8 +465,7 @@ stat_ref:
 	close(fd);
 	strbuf_rtrim(&sb_contents);
 	buf = sb_contents.buf;
-	if (starts_with(buf, "ref:")) {
-		buf += 4;
+	if (skip_prefix(buf, "ref:", &buf)) {
 		while (isspace(*buf))
 			buf++;
 

remote-curl.c

@@ -1255,8 +1255,9 @@ static void parse_push(struct strbuf *buf)
 	int ret;
 
 	do {
-		if (starts_with(buf->buf, "push "))
-			argv_array_push(&specs, buf->buf + 5);
+		const char *arg;
+		if (skip_prefix(buf->buf, "push ", &arg))
+			argv_array_push(&specs, arg);
 		else
 			die(_("http transport does not support %s"), buf->buf);
 

sha1-name.c

@@ -908,14 +908,9 @@ static int get_oid_basic(struct repository *r, const char *str, int len,
 				real_ref, flags, at_time, nth, oid, NULL,
 				&co_time, &co_tz, &co_cnt)) {
 			if (!len) {
-				if (starts_with(real_ref, "refs/heads/")) {
-					str = real_ref + 11;
-					len = strlen(real_ref + 11);
-				} else {
-					/* detached HEAD */
+				if (!skip_prefix(real_ref, "refs/heads/", &str))
 					str = "HEAD";
-					len = 4;
-				}
+				len = strlen(str);
 			}
 			if (at_time) {
 				if (!(flags & GET_OID_QUIETLY)) {

transport-helper.c

@@ -404,11 +404,12 @@ static int fetch_with_fetch(struct transport *transport,
 	sendline(data, &buf);
 
 	while (1) {
+		const char *name;
+
 		if (recvline(data, &buf))
 			exit(128);
 
-		if (starts_with(buf.buf, "lock ")) {
-			const char *name = buf.buf + 5;
+		if (skip_prefix(buf.buf, "lock ", &name)) {
 			if (transport->pack_lockfile)
 				warning(_("%s also locked %s"), data->name, name);
 			else