[Updater] Automatically run `git merge`

Test: updater.sh update xz-java
Change-Id: I3924b5651f2fac3c5d7493964b570f2f39bc4777

git_updater.py

@@ -129,10 +129,4 @@ class GitUpdater():
                 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)

git_utils.py

@@ -113,3 +113,8 @@ COMMIT_RE = re.compile(COMMIT_PATTERN)
 def is_commit(commit):
     """Whether a string looks like a SHA1 hash."""
     return bool(COMMIT_RE.match(commit))
+
+
+def merge(proj_path, branch):
+    """Merges a branch."""
+    _run(['git', 'merge', branch], cwd=proj_path)