Also, fix assertion that fires if all the reviews are
on the same day (first_day == last_day)

Also, don't print backtrace if no arguments at all are
specified.

Review URL: https://codereview.chromium.org/797523002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@293351 0039d316-1c4b-4281-b951-d872f2087c98