[gerrit] Fix encoding in ChangeEdit

gerrit_util.ChangeEdit was passing the result of b64encode into a %s
format, which was printing the contents with a bytes prefix (b'...').

Decode the bytes object back to a str object before passing it to '%s'

Bug: v8:12849
Change-Id: I8b54b427bbbe8b914444d7486b5a8afbd743de70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3683382
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>

gerrit_util.py

@@ -775,7 +775,8 @@ def ChangeEdit(host, change, path, data):
   path = 'changes/%s/edit/%s' % (change, urllib.parse.quote(path, ''))
   body = {
       'binary_content':
-      'data:text/plain;base64,%s' % base64.b64encode(data.encode('utf-8'))
+      'data:text/plain;base64,%s' %
+      base64.b64encode(data.encode('utf-8')).decode('utf-8')
   }
   conn = CreateHttpConn(host, path, reqtype='PUT', body=body)
   return ReadHttpJsonResponse(conn, accept_statuses=(204, 409))