fetchFromSourcehut: expose `gitRepoUrl` to consumers

notable consumers include `unstableGitUpdater`.
other git-like fetchers (`fetchFromGitHub`) already do this.

pkgs/build-support/fetchsourcehut/default.nix

···
 assert (lib.assertOneOf "vc" vc [ "hg" "git" ]);
 
 let
-  baseUrl = "https://${vc}.${domain}/${owner}/${repo}";
+  urlFor = resource: "https://${resource}.${domain}/${owner}/${repo}";
+  baseUrl = urlFor vc;
   baseArgs = {
     inherit name;
   } // removeAttrs args [
···
         postFetch = optionalString (vc == "hg") ''
           rm -f "$out/.hg_archival.txt"
         ''; # impure file; see #12002
+        passthru = {
+          gitRepoUrl = urlFor "git";
+        };
       };
     };
   };