Improved backwards compatibility hack for git URLs using dir=... attribute

(cherry picked from commit 43b01b6790)

src/libfetchers/git.cc

@@ -496,6 +496,36 @@ struct GitInputScheme : InputScheme
                    Git interprets them as part of the file name. So get
                    rid of them. */
                 url.query.clear();
+            /* Backward compatibility hack: In old versions of Nix, if you had
+               a flake input like
+
+                 inputs.foo.url = "git+https://foo/bar?dir=subdir";
+
+               it would result in a lock file entry like
+
+                 "original": {
+                   "dir": "subdir",
+                   "type": "git",
+                   "url": "https://foo/bar?dir=subdir"
+                 }
+
+               New versions of Nix remove `?dir=subdir` from the `url` field,
+               since the subdirectory is intended for `FlakeRef`, not the
+               fetcher (and specifically the remote server), that is, the
+               flakeref is parsed into
+
+                 "original": {
+                   "dir": "subdir",
+                   "type": "git",
+                   "url": "https://foo/bar"
+                 }
+
+               However, new versions of nix parsing old flake.lock files would pass the dir=
+               query parameter in the "url" attribute to git, which will then complain.
+
+               For this reason, we filtering the `dir` query parameter from the URL
+               before passing it to git. */
+            url.query.erase("dir");
             repoInfo.location = url;
         }