fetchsvn: fixup name guessing after #23851

I can't see any unfixed differences due to that PR,
at least in nix-env -qa output for the four Hydra platforms.

pkgs/build-support/fetchsvn/default.nix

@@ -8,7 +8,9 @@ let
       fst = head;
       snd = l: head (tail l);
       trd = l: head (tail (tail l));
-      path_ = reverseList (splitString "/" url);
+      path_ =
+        (p: if head p == "" then tail p else p) # ~ drop final slash if any
+        (reverseList (splitString "/" url));
       path = [ (removeSuffix "/" (head path_)) ] ++ (tail path_);
     in
       # ../repo/trunk -> repo