fix(npins): fetch dependencies from correct ref

We would previously get this error when fetching on new machines

… while fetching the input 'git+https://gerrit.wikimedia.org/r/mediawiki/extensions/AdvancedSearch?rev=398c9fa782843d8b3aeaa5ebb1c1b3db35c3382f'

error: Cannot find Git revision '398c9fa782843d8b3aeaa5ebb1c1b3db35c3382f' in ref 'refs/heads/master' of repository 'https://gerrit.wikimedia.org/r/mediawiki/extensions/AdvancedSearch'! Please make sure that the rev exists on the ref you've specified or add allRefs = true; to fetchGit.

Now, this is what I'd expect - because we're not meant to be fetching
master of this extension - we're meant to be fetching a branch! This
seems to be an issue with npins' fetchgit call, which misses out the ref

Due to a nilla bug, fetching dependencies is required for evaluation
so this prevented any evaluation

packetmix/npins/default.nix

···
                 url,
                 submodules,
                 rev,
+                branch ? null,
                 name,
                 narHash,
               }:
-              pkgs.fetchgit {
+              pkgs.fetchgit ({
                 inherit url rev name;
                 fetchSubmodules = submodules;
                 hash = narHash;
-              };
+              } // (if branch == null then {} else {
+                ref = "refs/heads/${branch}";
+              }));
           };
 
       # Dispatch to the correct code path based on the type
···
       repository,
       revision,
       url ? null,
+      branch ? null,
       submodules,
       hash,
       ...
···
           "${if matched == null then "source" else builtins.head matched}${appendShort}";
         name = urlToName url revision;
       in
-      fetchGit {
+      fetchGit ({
         rev = revision;
         narHash = hash;
 
         inherit name submodules url;
-      };
+      } // (if branch == null then {} else {
+        ref = "refs/heads/${branch}";
+      }));
 
   mkPyPiSource =
     { fetchurl, ... }: