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/ssh-shallow/default.nix

@@ -0,0 +1,52 @@
+{
+  description = "can fetch a git repo via ssh using shallow=1";
+  script = ''
+    # add a file to the repo
+    client.succeed(f"""
+      echo chiang-mai > {repo.path}/thailand \
+      && {repo.git} add thailand \
+      && {repo.git} commit -m 'commit1'
+    """)
+
+    # memoize the revision
+    rev1 = client.succeed(f"""
+      {repo.git} rev-parse HEAD
+    """).strip()
+
+    # push to the server
+    client.succeed(f"""
+      {repo.git} push origin-ssh main
+    """)
+
+    fetchGit_expr = f"""
+      builtins.fetchGit {{
+        url = "{repo.remote_ssh}";
+        rev = "{rev1}";
+        shallow = true;
+      }}
+    """
+
+    # fetch the repo via nix
+    fetched1 = client.succeed(f"""
+      nix eval --impure --raw --expr '({fetchGit_expr}).outPath'
+    """)
+
+    # check if the committed file is there
+    client.succeed(f"""
+      test -f {fetched1}/thailand
+    """)
+
+    # check if the revision is the same
+    rev1_fetched = client.succeed(f"""
+      nix eval --impure --raw --expr '({fetchGit_expr}).rev'
+    """).strip()
+    assert rev1 == rev1_fetched, f"rev1: {rev1} != rev1_fetched: {rev1_fetched}"
+
+    # check if revCount is 1
+    revCount1 = client.succeed(f"""
+      nix eval --impure --expr '({fetchGit_expr}).revCount'
+    """).strip()
+    print(f"revCount1: {revCount1}")
+    assert revCount1 == '0', f"rev count is not 0 but {revCount1}"
+  '';
+}