Print filenames for objects in status output with '--filenames' option

[jmurty]

The '--filenames' option makes it easy to view the filename represented
by a git-fat object reference, at the cost of a slight performance
and memory hit compared to the plain git-fat status command.

This option is most helpful when you are thinking about running
git-fat gc to clean up some garbage/unreferenced objects, so you
can check what you are about to delete.

• Add referenced_objects_with_filenames() method that (optionally)
  stores file name data while looking up git-fat referenced objects.
• Refactor referenced_objects() method to use the above method while
  providing existing interface.
• If '--filenames' option is given to the status command, print
  filename(s) next to git-fat object hash values.

[abraithwaite]

Hey @jmurty, I did something similar to this in our fork with git fat list.

Your solution of storing hash->filename strings in a dict is the one I tried first too, but you quickly run out of memory for medium to large sized repositories. The way we implemented it is running through rev-list twice which isn't ideal, but better than nothing at all.

[jmurty]

Thanks for the feedback @abraithwaite.

I have just added a small improvement to the proposed feature to clear out uninteresting filenames during processing, which should massively reduce the memory consumption. Or at least use no more memory than is really necessary to store all the filenames of interest.

Are you able to test this improvement against a medium- or large-size repository to see if it survives?

[abraithwaite]

I haven't tried this yet, but it looks like this part will fail on files with spaces.