Split brain: Make prep_ud be able to make a different dir (nfc)

We will want this later.  No users yet, so no functional change.

diff --git a/dgit b/dgit
index d59d2f5f17d7652131628ee502f0c667e100ed8b..2069298b906741e9d6f2a93f5afe793ccf5146c4 100755 (executable)
--- a/dgit
+++ b/dgit
@@ -1209,10 +1209,12 @@ our ($dsc_hash,$lastpush_hash);
 
 our $ud = '.git/dgit/unpack';
 
-sub prep_ud () {
-    rmtree($ud);
+sub prep_ud (;$) {
+    my ($d) = @_;
+    $d //= $ud;
+    rmtree($d);
     mkpath '.git/dgit';
-    mkdir $ud or die $!;
+    mkdir $d or die $!;
 }
 
 sub mktree_in_ud_here () {