make commit_tree a library function

Until now, this has been part of the commit-tree builtin.
However, it is already used by other builtins (like commit,
merge, and notes), and it would be useful to access it from
library code.

The check_valid helper has to come along, too, but is given
a more library-ish name of "assert_sha1_type".

Otherwise, the code is unchanged. There are still a few
rough edges for a library function, like printing the utf8
warning to stderr, but we can address those if and when they
come up as inappropriate.

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

commit.c

@@ -790,3 +790,58 @@ struct commit_list *reduce_heads(struct commit_list *heads)
 	free(other);
 	return result;
 }
+
+static const char commit_utf8_warn[] =
+"Warning: commit message does not conform to UTF-8.\n"
+"You may want to amend it after fixing the message, or set the config\n"
+"variable i18n.commitencoding to the encoding your project uses.\n";
+
+int commit_tree(const char *msg, unsigned char *tree,
+		struct commit_list *parents, unsigned char *ret,
+		const char *author)
+{
+	int result;
+	int encoding_is_utf8;
+	struct strbuf buffer;
+
+	assert_sha1_type(tree, OBJ_TREE);
+
+	/* Not having i18n.commitencoding is the same as having utf-8 */
+	encoding_is_utf8 = is_encoding_utf8(git_commit_encoding);
+
+	strbuf_init(&buffer, 8192); /* should avoid reallocs for the headers */
+	strbuf_addf(&buffer, "tree %s\n", sha1_to_hex(tree));
+
+	/*
+	 * NOTE! This ordering means that the same exact tree merged with a
+	 * different order of parents will be a _different_ changeset even
+	 * if everything else stays the same.
+	 */
+	while (parents) {
+		struct commit_list *next = parents->next;
+		strbuf_addf(&buffer, "parent %s\n",
+			sha1_to_hex(parents->item->object.sha1));
+		free(parents);
+		parents = next;
+	}
+
+	/* Person/date information */
+	if (!author)
+		author = git_author_info(IDENT_ERROR_ON_NO_NAME);
+	strbuf_addf(&buffer, "author %s\n", author);
+	strbuf_addf(&buffer, "committer %s\n", git_committer_info(IDENT_ERROR_ON_NO_NAME));
+	if (!encoding_is_utf8)
+		strbuf_addf(&buffer, "encoding %s\n", git_commit_encoding);
+	strbuf_addch(&buffer, '\n');
+
+	/* And add the comment */
+	strbuf_addstr(&buffer, msg);
+
+	/* And check the encoding */
+	if (encoding_is_utf8 && !is_utf8(buffer.buf))
+		fprintf(stderr, commit_utf8_warn);
+
+	result = write_sha1_file(buffer.buf, buffer.len, commit_type, ret);
+	strbuf_release(&buffer);
+	return result;
+}