[Updater] Automatically run `git merge`

Test: updater.sh update xz-java
Change-Id: I3924b5651f2fac3c5d7493964b570f2f39bc4777
diff --git a/git_updater.py b/git_updater.py
index 4c878c8..15f0360 100644
--- a/git_updater.py
+++ b/git_updater.py
@@ -129,10 +129,4 @@
                 self.merge_from, len(commits), upstream_branch))
 
         self._write_metadata(self.proj_path)
-        print("""
-This tool only updates METADATA. Run the following command to update:
-    git merge {merge_branch}
-
-To check all local changes:
-    git diff {merge_branch} HEAD
-""".format(merge_branch=self.merge_from))
+        git_utils.merge(self.proj_path, self.merge_from)