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 <[e-mail]>
Acked-by: Jan Engelhardt <[e-mail]>
Signed-off-by: Sam Ravnborg <[e-mail]>

scripts/setlocalversion

···
 if head=`git rev-parse --verify HEAD 2>/dev/null`; then
 	# Do we have an untagged version?
 	if git name-rev --tags HEAD | grep -E '^HEAD[[:space:]]+(.*~[0-9]*|undefined)$' > /dev/null; then
-	        git describe | awk -F- '{printf("-%05d-%s", $(NF-1),$(NF))}'
+		if tag=`git describe 2>/dev/null`; then
+			echo $tag | awk -F- '{printf("-%05d-%s", $(NF-1),$(NF))}'
+		fi
 	fi
 
 	# Are there uncommitted changes?