fetchTree: shallow git fetching by default

Motivation:
make git fetching more efficient for most repos by default

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

@@ -0,0 +1,45 @@
+{
+  description = "fetchTree fetches git repos shallowly by default";
+  script = ''
+    # purge nix git cache to make sure we start with a clean slate
+    client.succeed("rm -rf ~/.cache/nix")
+
+    # add two commits to the repo:
+    #   - one with a large file (2M)
+    #   - another one making the file small again
+    client.succeed(f"""
+      dd if=/dev/urandom of={repo.path}/thailand bs=1M count=2 \
+      && {repo.git} add thailand \
+      && {repo.git} commit -m 'commit1' \
+      && echo 'ThaigerSprint' > {repo.path}/thailand \
+      && {repo.git} add thailand \
+      && {repo.git} commit -m 'commit2' \
+      && {repo.git} push origin main
+    """)
+
+    # memoize the revision
+    commit2_rev = client.succeed(f"""
+      {repo.git} rev-parse HEAD
+    """).strip()
+
+    # construct the fetcher call
+    fetchGit_expr = f"""
+      builtins.fetchTree {{
+        type = "git";
+        url = "{repo.remote}";
+        rev = "{commit2_rev}";
+      }}
+    """
+
+    # fetch the repo via nix
+    fetched1 = client.succeed(f"""
+      nix eval --impure --raw --expr '({fetchGit_expr}).outPath'
+    """)
+
+    # check that the size of ~/.cache/nix is less than 1M
+    cache_size = client.succeed("""
+      du -s ~/.cache/nix
+    """).strip().split()[0]
+    assert int(cache_size) < 1024, f"cache size is {cache_size}K which is larger than 1M"
+  '';
+}