version: Print git id if present

If criu is built from git repo print git commit id, convenient for debug.

For example

 | $ ./criu -V
 | Version: 0.7
 | GitID: 7985251

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>