git module: support windows

remove dependence on `pipes.quote` in favor of
`subprocess.list2cmdline`, which is cross-platform. This permits the
git module to be used by windows minions.

`list2cmdline` quotes a little differently than `pipes.quote`, but
produces a safe result:

        >>> x = 'A & "B"'
	>>> print pipes.quote(x)
	'A & "B"'
	>>> print subprocess.list2cmdline([x])
	"A & \"B\""
git state: use cross-platform path seperator

use `os.path.join` instead of a format string
git state: more cross-platform paths

use os.path.join instead of string building
use explicit boolean in `__virtual__`

Returning a non-boolean value will rename the package
Merge pull request #12304 from AccelerationNet/git-windows

git module: support windows