We're going to need to scan trees with ls-tree in the git package in the
future, and we can't call into the lfs package because of import loops,
so let's move the scanner to the git package.
While we're at it, let's make two important changes. First, let's
remove the blob size check, since we're going to want this functionality
in order to read all blobs, not just small ones. As part of that, move
that check into the place where we use the output of the scanner so we
don't lose this check.
The other check is to change the name Sha1 to Oid, since we now support
SHA-256 repos as well as SHA-1 repos.
Move the tests and some of the helper functions to the new package as
well.
1e41bbffbb