Immediately save the note when changing

Earlier we would wait till we got the 'mutex' to make a change, however
with the way libgit2 works, during a git push/pull other git operations
such as add/rm/commit block.

So performing a save, while the push/pull is occuring - might have
resulted in a note being lost

Related to #124

lib/core/git_repo.dart

@@ -36,7 +36,6 @@ class GitNoteRepository {
   }
 
   Future<NoteRepoResult> _addNote(Note note, String commitMessage) async {
-    await note.save();
     await _gitRepo.add(".");
     await _gitRepo.commit(
       message: commitMessage,