diff --git a/builtin-commit-tree.c b/builtin-commit-tree.c
index 3881f6c2f5..7a9a309be0 100644
--- a/builtin-commit-tree.c
+++ b/builtin-commit-tree.c
@@ -45,41 +45,19 @@ static const char commit_utf8_warn[] =
 "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 cmd_commit_tree(int argc, const char **argv, const char *prefix)
+int commit_tree(const char *msg, unsigned char *tree,
+		struct commit_list *parents, unsigned char *ret)
 {
-	int i;
-	struct commit_list *parents = NULL;
-	unsigned char tree_sha1[20];
-	unsigned char commit_sha1[20];
-	struct strbuf buffer;
 	int encoding_is_utf8;
+	struct strbuf buffer;
 
-	git_config(git_default_config, NULL);
-
-	if (argc < 2)
-		usage(commit_tree_usage);
-	if (get_sha1(argv[1], tree_sha1))
-		die("Not a valid object name %s", argv[1]);
-
-	check_valid(tree_sha1, OBJ_TREE);
-	for (i = 2; i < argc; i += 2) {
-		unsigned char sha1[20];
-		const char *a, *b;
-		a = argv[i]; b = argv[i+1];
-		if (!b || strcmp(a, "-p"))
-			usage(commit_tree_usage);
-
-		if (get_sha1(b, sha1))
-			die("Not a valid object name %s", b);
-		check_valid(sha1, OBJ_COMMIT);
-		new_parent(lookup_commit(sha1), &parents);
-	}
+	check_valid(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_sha1));
+	strbuf_addf(&buffer, "tree %s\n", sha1_to_hex(tree));
 
 	/*
 	 * NOTE! This ordering means that the same exact tree merged with a
@@ -102,14 +80,47 @@ int cmd_commit_tree(int argc, const char **argv, const char *prefix)
 	strbuf_addch(&buffer, '\n');
 
 	/* And add the comment */
-	if (strbuf_read(&buffer, 0, 0) < 0)
-		die("git-commit-tree: read returned %s", strerror(errno));
+	strbuf_addstr(&buffer, msg);
 
 	/* And check the encoding */
 	if (encoding_is_utf8 && !is_utf8(buffer.buf))
 		fprintf(stderr, commit_utf8_warn);
 
-	if (!write_sha1_file(buffer.buf, buffer.len, commit_type, commit_sha1)) {
+	return write_sha1_file(buffer.buf, buffer.len, commit_type, ret);
+}
+
+int cmd_commit_tree(int argc, const char **argv, const char *prefix)
+{
+	int i;
+	struct commit_list *parents = NULL;
+	unsigned char tree_sha1[20];
+	unsigned char commit_sha1[20];
+	struct strbuf buffer = STRBUF_INIT;
+
+	git_config(git_default_config, NULL);
+
+	if (argc < 2)
+		usage(commit_tree_usage);
+	if (get_sha1(argv[1], tree_sha1))
+		die("Not a valid object name %s", argv[1]);
+
+	for (i = 2; i < argc; i += 2) {
+		unsigned char sha1[20];
+		const char *a, *b;
+		a = argv[i]; b = argv[i+1];
+		if (!b || strcmp(a, "-p"))
+			usage(commit_tree_usage);
+
+		if (get_sha1(b, sha1))
+			die("Not a valid object name %s", b);
+		check_valid(sha1, OBJ_COMMIT);
+		new_parent(lookup_commit(sha1), &parents);
+	}
+
+	if (strbuf_read(&buffer, 0, 0) < 0)
+		die("git-commit-tree: read returned %s", strerror(errno));
+
+	if (!commit_tree(buffer.buf, tree_sha1, parents, commit_sha1)) {
 		printf("%s\n", sha1_to_hex(commit_sha1));
 		return 0;
 	}
diff --git a/builtin.h b/builtin.h
index 2b01feabcb..05ee56f21b 100644
--- a/builtin.h
+++ b/builtin.h
@@ -3,6 +3,8 @@
 
 #include "git-compat-util.h"
 #include "strbuf.h"
+#include "cache.h"
+#include "commit.h"
 
 extern const char git_version_string[];
 extern const char git_usage_string[];
@@ -14,6 +16,8 @@ extern void prune_packed_objects(int);
 extern int read_line_with_nul(char *buf, int size, FILE *file);
 extern int fmt_merge_msg(int merge_summary, struct strbuf *in,
 	struct strbuf *out);
+extern int commit_tree(const char *msg, unsigned char *tree,
+		struct commit_list *parents, unsigned char *ret);
 
 extern int cmd_add(int argc, const char **argv, const char *prefix);
 extern int cmd_annotate(int argc, const char **argv, const char *prefix);