setlocalversion: do not describe if there is nothing to describe

Jan Engelhardt wrote:
> Just a note that when you run git-describe, you should probably quiten it.
>
> fatal: cannot describe 'bd7364a0fd5a4a2878fe4a224be1b142a4e6698e'
>
> This happens when tags are not present, which can happen if Linus's tree
> is sent upwards again, IOW:
>
>  machine1$  git-clone torvalds/linux-2.6.git
>  machine1$  git push elsewhere master
>
>  machine2$  git-clone elsewhere:/linux
>  machine2$  git-describe HEAD
>  fatal: cannot describe that

Signed-off-by: Sebastian Siewior <sebastian@breakpoint.cc>
Acked-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>