Introduced sha256 support for git-sizer

[fcharlie]

Recreate for #109

[JamesB192]

Seems to work with a modest repo.

berg2 $ ~/go/bin/git-sizer
Processing blobs: 101555                        
Processing trees: 69385                        
Processing commits: 35267                        
Matching commits to trees: 35267                        
Processing annotated tags: 0                        
Processing references: 8                        
No problems above the current threshold were found

[bk2204]

This mostly looks fine to me, but I see some changes I think are extraneous here that either should be explained or backed out. With those fixed, I think this should be fine to merge.

[bk2204]

By the way, my apologies for the long delay in getting to this. I'll try to see to it that we get to PRs on this repo in a more timely manner.