abuild.in: don't fail if git describe fails

`git describe` by default looks for tags, but `git clone` does not clone
tags by default which causes failures on travis currently.

Also redirect `git describe` errors to /dev/null while being here.

abuild.in

@@ -1548,7 +1548,7 @@ update_abuildrepo_index() {
 	subpkg_unset
 
 	if [ -z "$DESCRIPTION" ]; then
-		DESCRIPTION="$repo $(cd $startdir && $git describe)"
+		DESCRIPTION="$repo $(cd $startdir && $git describe 2>/dev/null || true)"
 	fi
 
 	for i in $allarch; do