Make git-try use trychange for both SVN and HTTP try
server submits.  Simplifies git-try and reduces chance
of differences in behavior between SVN and HTTP try
job submits.

Exit early if the checkout's configured email address
does not pass validation.

Simplify trychange import.

BUG=none
TEST=git-try submits a job to the try server
Review URL: http://codereview.chromium.org/173394

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@24315 0039d316-1c4b-4281-b951-d872f2087c98