repo-patch: Download patch files from Gerrit

This commit adds a utility to download multiple patch files from Gerrit
by a change list query.

Test: repo_patch.py -g [gerrit] [query]
Change-Id: I3685c48d2ea4e5a9fa1915c4e31bc6ebb2d4ed97

tools/repo_pull/gerrit.py

@@ -19,6 +19,7 @@
 from __future__ import print_function
 
 import argparse
+import base64
 import json
 import os
 import sys
@@ -158,6 +159,19 @@ def set_review(url_opener, gerrit_url, change_id, labels, message):
         response_file.close()
 
 
+def get_patch(url_opener, gerrit_url, change_id, revision_id='current'):
+    """Download the patch file."""
+
+    url = '{}/a/changes/{}/revisions/{}/patch'.format(
+        gerrit_url, change_id, revision_id)
+
+    response_file = url_opener.open(url)
+    try:
+        return base64.b64decode(response_file.read())
+    finally:
+        response_file.close()
+
+
 def _parse_args():
     """Parse command line options."""
     parser = argparse.ArgumentParser()