flake: use builtins.fetchTree to shallow-clone grammar repos

Here we perform a shallow fetch using builtins.fetchTree. In order
to make this work, we need to specify the `ref' for any repository
that doesn't have `master' as its default branch (I'm not sure why
this limitation exists since we don't need this when performing
the shallow fetch in `--grammar build')

This `ref' field is ignored by helix, so I have left it undocumented
for now, but I could be open to documenting it.

flake.nix

@@ -12,17 +12,9 @@
       inputs.nixpkgs.follows = "nixpkgs";
       inputs.rustOverlay.follows = "rust-overlay";
     };
-    # NOTE: the flake looks like it is hanging when it pulls this input because
-    # the submodules take a long time to clone. This will be fixed in #1659.
-    helix = {
-      url = "https://github.com/helix-editor/helix.git";
-      type = "git";
-      submodules = true;
-      flake = false;
-    };
   };
 
-  outputs = inputs@{ nixCargoIntegration, helix, ... }:
+  outputs = inputs@{ nixCargoIntegration, ... }:
     nixCargoIntegration.lib.makeOutputs {
       root = ./.;
       renameOutputs = { "helix-term" = "helix"; };

grammars.nix

@@ -13,10 +13,12 @@ let
   gitGrammars = builtins.filter isGitGrammar languagesConfig.grammar;
   buildGrammar = grammar:
     let
-      source = builtins.fetchGit {
+      source = builtins.fetchTree {
+        type = "git";
         url = grammar.source.git;
         rev = grammar.source.rev;
-        allRefs = true;
+        ref = grammar.source.ref or "HEAD";
+        shallow = true;
       };
     in stdenv.mkDerivation rec {
       # see https://github.com/NixOS/nixpkgs/blob/fbdd1a7c0bc29af5325e0d7dd70e804a972eb465/pkgs/development/tools/parsing/tree-sitter/grammar.nix

languages.toml

@@ -73,7 +73,7 @@ indent = { tab-width = 2, unit = "  " }
 
 [[grammar]]
 name = "protobuf"
-source = { git = "https://github.com/yusdacra/tree-sitter-protobuf", rev = "19c211a01434d9f03efff99f85e19f967591b175" }
+source = { git = "https://github.com/yusdacra/tree-sitter-protobuf", rev = "19c211a01434d9f03efff99f85e19f967591b175"}
 
 [[language]]
 name = "elixir"
@@ -88,7 +88,7 @@ indent = { tab-width = 2, unit = "  " }
 
 [[grammar]]
 name = "elixir"
-source = { git = "https://github.com/elixir-lang/tree-sitter-elixir", rev = "f5d7bda543da788bd507b05bd722627dde66c9ec" }
+source = { git = "https://github.com/elixir-lang/tree-sitter-elixir", rev = "f5d7bda543da788bd507b05bd722627dde66c9ec"  }
 
 [[language]]
 name = "fish"