vc-git.el: Take into account the destination name in vc-checkout.

vc-git.el: Take into account the destination name in vc-checkout.

This is necessary for vc-version-other-window. Based on a patch by Sam
Vilain <sam.vilain@catalyst.net.nz>.

Currently, the vc-git-checkout function uses `git checkout' to fetch a
file from the git repository to the working copy.  However, it is
completely ignoring the input argument that specifies the destination
file.  `git-checkout' does not support specifying this, so we have to
use `git-cat-file', capture the output in a buffer and then save it.

Signed-off-by: Alexandre Julliard <julliard@winehq.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>