Add "git cl set_commit" command to set the flag more easily.
Add --commit to "git cl upload" to streamline workflow even more.
Continue conversion to Rietveld object.

R=dpranke@chromium.org
BUG=
TEST=

Review URL: http://codereview.chromium.org/7084037

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@87253 0039d316-1c4b-4281-b951-d872f2087c98