Fixed a minor bug related to fast check of digest file.
Completed the removal of the gc function - had forgotten to take the cmd out of the list of available commands in the previous commit.
Greatly improved the performance of referenced_objects
- Added support for 'local', which allows fine grained push to be implemented. Thus, it is no longer
  necessary for git-fat to run rsync on the entire content of .git/fat/objects => faster pushes.
- Enhanced smudge filter to retrieve only what HEAD references (no history) => faster update of
  your tree. As a result, you don't have to run 'git fat pull' which would pull in all fat files
  that HEAD (including history) references. You can for instance just do 'git pull' and the smudge
  filter will do the rest. Note: Not doing 'git fat pull' up front comes with a performance hit
  if ssh is set up to run rsync over because each new fat file the smudge filter needs requires
  a new ssh connection to be established.
- local, if set up appropriately, allows sharing of fat files across multiple clones of the same
  repo => more efficient use of disk space.
- Also now running 'git push' from within 'git fat push' in order to streamline push operation.
  'git push' is executed *after* fat files are pushed successfully.
- Also now running 'git pull' from within 'git fat pull' in order to streamline pull operation.
  'git pull' is executed *before* fat files are pulled.
Fixed fast decode operation in repos that use sparse-checkout
Restored gc functionality as it is now pretty cheap to restore what was 'deleted' due to support for 'local'
Added help command for a quick reference
Added some questions (TODO) regarding orphan_files - Perhaps some optimization is possible.
Note for a bug is recorded as a TODO - should be fixed, but not urgent as it is not too harmful