commit: convert commit_tree* to object_id

Convert the definitions and declarations of commit_tree and
commit_tree_extended to use struct object_id and adjust all usages of
these functions.

Signed-off-by: Patryk Obara <patryk.obara@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/commit.h b/commit.h
index 425f402..0fb8271 100644
--- a/commit.h
+++ b/commit.h
@@ -262,14 +262,15 @@ extern void append_merge_tag_headers(struct commit_list *parents,
 				     struct commit_extra_header ***tail);
 
 extern int commit_tree(const char *msg, size_t msg_len,
-		       const unsigned char *tree,
-		       struct commit_list *parents, unsigned char *ret,
+		       const struct object_id *tree,
+		       struct commit_list *parents, struct object_id *ret,
 		       const char *author, const char *sign_commit);
 
 extern int commit_tree_extended(const char *msg, size_t msg_len,
-				const unsigned char *tree,
-				struct commit_list *parents, unsigned char *ret,
-				const char *author, const char *sign_commit,
+				const struct object_id *tree,
+				struct commit_list *parents,
+				struct object_id *ret, const char *author,
+				const char *sign_commit,
 				struct commit_extra_header *);
 
 extern struct commit_extra_header *read_commit_extra_headers(struct commit *, const char **);