Auto-roll script: Add dirty tree check and git pull

tools/autoroller/roll_chromium_revision.py

Index: tools/autoroller/roll_chromium_revision.py
diff --git a/tools/autoroller/roll_chromium_revision.py b/tools/autoroller/roll_chromium_revision.py
index 57937ed82ee3e6042245524bd3271a9bd940b433..4dd7809b066e931216abf7d43da3f6f432e55922 100755
--- a/tools/autoroller/roll_chromium_revision.py
+++ b/tools/autoroller/roll_chromium_revision.py
@@ -298,6 +298,8 @@ def _CreateRollBranch(dry_run):
     if not dry_run:
       sys.exit(-1)
 
+  logging.info('Updating master branch...')
+  _RunCommand(['git', 'pull'])

[pbos-webrtc, 2015/06/09 09:34:31]

Do you want to do this under a dry run? Also is there any sanity check possible
that you've not applied commits onto master?

[kjellander_webrtc, 2015/06/09 10:51:04]

I thought it shouldn't never hurt your checkout to run it, but sure, let's not
do it.
I added a check for a dirty tree, but I think it's hard to check that no local
commits have been made on master, let's skip that.

   logging.info('Creating roll branch: %s', ROLL_BRANCH_NAME)
   if not dry_run:
     _RunCommand(['git', 'checkout', '-b', ROLL_BRANCH_NAME])