Improve the FileSystemObjectSink interface

More invariants are enforced in the type, and less state needs to be
stored in the main sink itself. The method here is roughly that known as
"session types".

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libutil/git.hh

@@ -13,12 +13,19 @@
 
 namespace nix::git {
 
+enum struct ObjectType {
+    Blob,
+    Tree,
+    //Commit,
+    //Tag,
+};
+
 using RawMode = uint32_t;
 
 enum struct Mode : RawMode {
     Directory = 0040000,
-    Executable = 0100755,
     Regular = 0100644,
+    Executable = 0100755,
     Symlink = 0120000,
 };
 
@@ -59,9 +66,34 @@ using Tree = std::map<std::string, TreeEntry>;
  */
 using SinkHook = void(const Path & name, TreeEntry entry);
 
+/**
+ * Parse the "blob " or "tree " prefix.
+ *
+ * @throws if prefix not recognized
+ */
+ObjectType parseObjectType(
+    Source & source,
+    const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
+
+void parseBlob(
+    FileSystemObjectSink & sink, const Path & sinkPath,
+    Source & source,
+    bool executable,
+    const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
+
+void parseTree(
+    FileSystemObjectSink & sink, const Path & sinkPath,
+    Source & source,
+    std::function<SinkHook> hook,
+    const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
+
+/**
+ * Helper putting the previous three `parse*` functions together.
+ */
 void parse(
     FileSystemObjectSink & sink, const Path & sinkPath,
     Source & source,
+    bool executable,
     std::function<SinkHook> hook,
     const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);