commit: move empty message checks to libgit

Move the functions that check for empty messages from bulitin/commit.c
to sequencer.c so they can be shared with other commands. The
functions are refactored to take an explicit cleanup mode and template
filename passed by the caller.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -691,6 +691,67 @@ static int run_git_commit(const char *defmsg, struct replay_opts *opts,
 	return run_command(&cmd);
 }
 
+static int rest_is_empty(const struct strbuf *sb, int start)
+{
+	int i, eol;
+	const char *nl;
+
+	/* Check if the rest is just whitespace and Signed-off-by's. */
+	for (i = start; i < sb->len; i++) {
+		nl = memchr(sb->buf + i, '\n', sb->len - i);
+		if (nl)
+			eol = nl - sb->buf;
+		else
+			eol = sb->len;
+
+		if (strlen(sign_off_header) <= eol - i &&
+		    starts_with(sb->buf + i, sign_off_header)) {
+			i = eol;
+			continue;
+		}
+		while (i < eol)
+			if (!isspace(sb->buf[i++]))
+				return 0;
+	}
+
+	return 1;
+}
+
+/*
+ * Find out if the message in the strbuf contains only whitespace and
+ * Signed-off-by lines.
+ */
+int message_is_empty(const struct strbuf *sb,
+		     enum commit_msg_cleanup_mode cleanup_mode)
+{
+	if (cleanup_mode == COMMIT_MSG_CLEANUP_NONE && sb->len)
+		return 0;
+	return rest_is_empty(sb, 0);
+}
+
+/*
+ * See if the user edited the message in the editor or left what
+ * was in the template intact
+ */
+int template_untouched(const struct strbuf *sb, const char *template_file,
+		       enum commit_msg_cleanup_mode cleanup_mode)
+{
+	struct strbuf tmpl = STRBUF_INIT;
+	const char *start;
+
+	if (cleanup_mode == COMMIT_MSG_CLEANUP_NONE && sb->len)
+		return 0;
+
+	if (!template_file || strbuf_read_file(&tmpl, template_file, 0) <= 0)
+		return 0;
+
+	strbuf_stripspace(&tmpl, cleanup_mode == COMMIT_MSG_CLEANUP_ALL);
+	if (!skip_prefix(sb->buf, tmpl.buf, &start))
+		start = sb->buf;
+	strbuf_release(&tmpl);
+	return rest_is_empty(sb, start - sb->buf);
+}
+
 static int is_original_commit_empty(struct commit *commit)
 {
 	const struct object_id *ptree_oid;