Teach status to recognize multiple files with identical contents

[ttaylorr]

This pull-request teaches the git lfs status command how to recognize multiple files that share the same contents. By modifying the behavior of lfs.ScanIndex, we now maintain a slice of file metadata associated with a given OID, instead of a one-to-one association (as before).

ScanIndex now only accepts unique pointer OIDs (even if there are multiple pointers for a given OID, in the case of many files sharing the same contents), and then resolves them all at once to multiple files, per the data stored in the indexFileMap.

While working on this, @technoweenie and I found what may be a Git core bug having to do with checking a file that is both in the index and working tree (i.e., it appears in both git diff-index and git diff-index --cached). I stashed a failing test that I wrote to demonstrate this behavior in this Gist.

As an aside, this PR is a temporary measure. Maintaining these extra caches is not a long-term solution that I'd like to keep, this is more of a hack to solve this bug quickly.

/cc @technoweenie @sinbad @rubyist

[sinbad]

I had to double-check the nil behaviour in a couple of cases but this looks fine. 👍

[ttaylorr]

Thanks for the review! 👋