It now defaults to origin/master when no branch is tracked.

TEST=git-try on untracked branch now doesn't result in an empty diff
BUG=none

Review URL: http://codereview.chromium.org/1739021

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@45935 0039d316-1c4b-4281-b951-d872f2087c98