support sha256 git repos

support sha256 git repos

Git will eventually switch to sha2 and there will not be one single
shaSize anymore, but two (40 and 64).

Changed all parsers for git plumbing output to support both sizes of
shas.

One potential problem this does not deal with is, if somewhere in
git-annex it reads two shas from different sources, and compares them
to see if they're the same sha, it would fail if they're sha1 and sha256
of the same value. I don't know if that will really be a concern.