'git try': a) Give users help when they ask for it, b) Don't start try job if you don't understand the command.
'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: bd64dd6fa6f3f0ed0c0666d1018379882b742947@54374 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools