'git try': a) Give users help when they ask for it, b) Don't start try job if you don't understand the command.

BUG=none
TEST=Make change in local sandbox and confirm proper things happen on "git try help", "git try xyzzy", and "git try -b linux."

Review URL: http://codereview.chromium.org/3075009

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@54374 0039d316-1c4b-4281-b951-d872f2087c98