fetchTree/fetchGit: re-enable shallow fetching

Add several tests for git fetching:
- shallow-cache-separation: can fetch the same repo shallowly and non-shallowly
- shallow-ignore-ref: ensure that ref gets ignored when shallow=true is set
- ssh-shallow: can fetch a git repo via ssh using shallow=1

tests/nixos/fetch-git/test-cases/shallow-ignore-ref/default.nix

@@ -0,0 +1,40 @@
+{
+  description = "ensure that ref gets ignored when shallow=true is set";
+  script = ''
+    # create branch1 off of main
+    client.succeed(f"""
+      echo chiang-mai > {repo.path}/thailand \
+      && {repo.git} add thailand \
+      && {repo.git} commit -m 'commit1' \
+      \
+      && {repo.git} checkout -b branch1 main \
+      && echo bangkok > {repo.path}/thailand \
+      && {repo.git} add thailand \
+      && {repo.git} commit -m 'commit2' \
+      \
+      && {repo.git} push origin --all
+    """)
+
+    # save the revisions
+    mainRev = client.succeed(f"""
+      {repo.git} rev-parse main
+    """).strip()
+    branch1Rev = client.succeed(f"""
+      {repo.git} rev-parse branch1
+    """).strip()
+
+    # Ensure that ref gets ignored when fetching shallowly.
+    # This would fail if the ref was respected, as branch1Rev is not on main.
+    client.succeed(f"""
+      nix eval --impure --raw --expr '
+        (builtins.fetchGit {{
+          url = "{repo.remote}";
+          rev = "{branch1Rev}";
+          ref = "main";
+          shallow = true;
+        }})
+      '
+    """)
+
+  '';
+}