Support building flakes from a shallow Git repo

Fixes #3756.

src/libexpr/flake/flakeref.cc

@@ -142,6 +142,9 @@ std::pair<FlakeRef, std::string> parseFlakeRefWithFragment(
                     parsedURL.query.insert_or_assign("dir", subdir);
                 }
 
+                if (pathExists(flakeRoot + "/.git/shallow"))
+                    parsedURL.query.insert_or_assign("shallow", "1");
+
                 return std::make_pair(
                     FlakeRef(Input::fromURL(parsedURL), get(parsedURL.query, "dir").value_or("")),
                     fragment);

src/libfetchers/git.cc

@@ -42,6 +42,8 @@ struct GitInputScheme : InputScheme
         for (auto &[name, value] : url.query) {
             if (name == "rev" || name == "ref")
                 attrs.emplace(name, value);
+            else if (name == "shallow")
+                attrs.emplace(name, Explicit<bool> { value == "1" });
             else
                 url2.query.emplace(name, value);
         }