Ref disambiguation, and barebones Log (doesn't correspond to any of git's logs in order)

Git/Reference.fs

@@ -0,0 +1,29 @@
+namespace Git
+
+
+type ReferenceUpdate =
+    {
+        Was : Hash option
+        Now : Hash
+    }
+
+[<RequireQualifiedAccess>]
+module Reference =
+    let write (r : Repository) (hash : Hash) (name : string) : ReferenceUpdate =
+        let refFile = r.Fs.Path.Combine ((Repository.refDir r).FullName, "heads", name) |> r.Fs.FileInfo.FromFileName
+        let was =
+            if refFile.Exists then
+                r.Fs.File.ReadAllText refFile.FullName
+                |> Hash.ofString
+                |> Some
+            else
+                do
+                    use _v = refFile.Create ()
+                    ()
+                None
+        r.Fs.File.WriteAllText (refFile.FullName, hash.ToString ())
+        {
+            Was = was
+            Now = hash
+        }
+