libgit2, GitRepo: Write thin packfiles

libgit2 didn't write thin ones, hence the patch.

This should improve performance on systems with weak I/O in ~/.cache,
especially in terms of operations per second, or where system calls
are slower. (macOS, VMs?)

src/libfetchers/git-utils.hh

@@ -80,6 +80,8 @@ struct GitRepo
 
     virtual ref<GitFileSystemObjectSink> getFileSystemObjectSink() = 0;
 
+    virtual void flush() = 0;
+
     virtual void fetch(
         const std::string & url,
         const std::string & refspec,