`git describe` outputs slightly more useful git version info: v1.5-132-ga10f39e

The motivation for this is that we'd like to begin enforcing criu version in
liblxc, but we'd like to enable people to use the git version of 1.6 (since
that has stuff like --ext-mount-map auto), which reports its version as 1.5 but
doesn't tell us the patchlevel as git describe does.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>