scripts/setlocalversion: make git describe output more reliable

When building for an embedded target using Yocto, we're sometimes
observing that the version string that gets built into vmlinux (and
thus what uname -a reports) differs from the path under /lib/modules/
where modules get installed in the rootfs, but only in the length of
the -gabc123def suffix. Hence modprobe always fails.

The problem is that Yocto has the concept of "sstate" (shared state),
which allows different developers/buildbots/etc. to share build
artifacts, based on a hash of all the metadata that went into building
that artifact - and that metadata includes all dependencies (e.g. the
compiler used etc.). That normally works quite well; usually a clean
build (without using any sstate cache) done by one developer ends up
being binary identical to a build done on another host. However, one
thing that can cause two developers to end up with different builds
[and thus make one's vmlinux package incompatible with the other's
kernel-dev p...