checkout problem in windows

agit-git-checkout does not work in Windows.
There are 2 reasons for it.

1. v:shell_error is still used with vimproc#system().
2. branch name returned from `git rev-parse` has trailing linebreak, 
    and git will say `error: pathspec 'master\n' did not match any file(s) known to git`

some other commands may fail by same reason.