Add close wappers for commit, tree, tag and blob

In the same spirit that git_repository_lookup is no longer available,
add wrappers so the users don't have to cast when closing their
objects.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>

include/git2/commit.h

@@ -53,6 +53,23 @@ GIT_INLINE(int) git_commit_lookup(git_commit **commit, git_repository *repo, con
 	return git_object_lookup((git_object **)commit, repo, id, GIT_OBJ_COMMIT);
 }
 
+/**
+ * Close an open commit
+ *
+ * This is a wrapper around git_object_close()
+ *
+ * IMPORTANT:
+ * It *is* necessary to call this method when you stop
+ * using a commit. Failure to do so will cause a memory leak.
+ *
+ * @param commit the commit to close
+ */
+
+GIT_INLINE(void) git_commit_close(git_commit *commit)
+{
+	return git_object_close((git_object *) commit);
+}
+
 /**
  * Get the id of a commit.
  *