Make "git prune" use the "--full" flag to git-fsck-cache

It's too dangerous not to.  We need to follow alternate object
directories etc, or we might say something is unreachable just because
we didn't look it up completely.

git-prune-script

@@ -14,7 +14,7 @@ do
     shift;
 done
 
-git-fsck-cache --cache --unreachable "$@" |
+git-fsck-cache --full --cache --unreachable "$@" |
 sed -ne '/unreachable /{
     s/unreachable [^ ][^ ]* //
     s|\(..\)|\1/|p