- Create logs URL for both gitiles and github.
- Include the number of commits being rolled in in the commit subject, which is
  always useful.
- Make the log inclusion conditional on the repository being rolled in and the
  number of commits.
- Checkout the new commit when using this script, otherwise this is surprising
  to users.
- Add flags to support more use cases.
- Use --quiet when calling git to reduce noise in the output.

R=spang@chromium.org,jochen@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1366493003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@296924 0039d316-1c4b-4281-b951-d872f2087c98