git-gui: corrected setup of git worktree under cygwin.

Under cygwin the _gitworktree variable needs to contain the Windows
style path string so the output provided by git rev-parse must
be converted from cygwin path style to native.

Reviewed-by: Jesse Welch <jesse.welch@baml.com>
Signed-off-by: John Patrick Murphy <john.murphy@baml.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>