Add new code review front end: "git review"

Adds an alias "git review" that calls a new script git_review.sh.  It
provides simple updates of new patchsets to the same review, commit message
reviewing by uploading the precise message for each patchset, and
auto-adding of the review URL to the commit message.

The existing "git dcommit" now also calls git_review.sh to mark the review
as committed with its final diff and a pointer to the online commit, as
well as remove the current review issue marker.

Review-URL: https://codereview.appspot.com/187770044