Add link to tracking issue

Author: edolstra

src/libfetchers/git.cc

@@ -439,7 +439,8 @@ struct GitInputScheme : InputScheme
             if (!isAbsolute(url.path)) {
                 warn(
                     "Fetching Git repository '%s', which uses a path relative to the current directory. "
-                    "This is not supported and will stop working in a future release.",
+                    "This is not supported and will stop working in a future release. "
+                    "See https://github.com/NixOS/nix/issues/12281 for details.",
                     url);
             }
             repoInfo.url = std::filesystem::absolute(url.path).string();