Git fetcher: Improve submodule handling

Instead of making a complete copy of the repo, fetching the
submodules, and writing the result to the store (which is all
superexpensive), we now fetch the submodules recursively using the Git
fetcher, and return a union accessor that "mounts" the accessors for
the submodules on top of the root accessor.

src/libfetchers/union-input-accessor.cc

@@ -0,0 +1,80 @@
+#include "union-input-accessor.hh"
+
+namespace nix {
+
+struct UnionInputAccessor : InputAccessor
+{
+    std::map<CanonPath, ref<InputAccessor>> mounts;
+
+    UnionInputAccessor(std::map<CanonPath, ref<InputAccessor>> _mounts)
+        : mounts(std::move(_mounts))
+    {
+        // Currently we require a root filesystem. This could be relaxed.
+        assert(mounts.contains(CanonPath::root));
+
+        // FIXME: should check that every mount point exists. Or we
+        // could return dummy parent directories automatically.
+    }
+
+    std::string readFile(const CanonPath & path) override
+    {
+        auto [accessor, subpath] = resolve(path);
+        return accessor->readFile(subpath);
+    }
+
+    bool pathExists(const CanonPath & path) override
+    {
+        auto [accessor, subpath] = resolve(path);
+        return accessor->pathExists(subpath);
+    }
+
+    Stat lstat(const CanonPath & path) override
+    {
+        auto [accessor, subpath] = resolve(path);
+        return accessor->lstat(subpath);
+    }
+
+    DirEntries readDirectory(const CanonPath & path) override
+    {
+        auto [accessor, subpath] = resolve(path);
+        return accessor->readDirectory(subpath);
+    }
+
+    std::string readLink(const CanonPath & path) override
+    {
+        auto [accessor, subpath] = resolve(path);
+        return accessor->readLink(subpath);
+    }
+
+    std::string showPath(const CanonPath & path) override
+    {
+        auto [accessor, subpath] = resolve(path);
+        return accessor->showPath(subpath);
+    }
+
+    std::pair<ref<InputAccessor>, CanonPath> resolve(CanonPath path)
+    {
+        // Find the nearest parent of `path` that is a mount point.
+        std::vector<std::string> ss;
+        while (true) {
+            auto i = mounts.find(path);
+            if (i != mounts.end()) {
+                auto subpath = CanonPath::root;
+                for (auto j = ss.rbegin(); j != ss.rend(); ++j)
+                    subpath.push(*j);
+                return {i->second, std::move(subpath)};
+            }
+
+            assert(!path.isRoot());
+            ss.push_back(std::string(*path.baseName()));
+            path.pop();
+        }
+    }
+};
+
+ref<InputAccessor> makeUnionInputAccessor(std::map<CanonPath, ref<InputAccessor>> mounts)
+{
+    return make_ref<UnionInputAccessor>(std::move(mounts));
+}
+
+}