fs: don't implicitly default to pygit2

[efiop]

get_fs doesn't pass a trie to the fs:

scmrepo/scmrepo/git/__init__.py

Line 300 in 28973b1

return GitFileSystem(scm=self, rev=rev)

which results in it always defaulting to pygit2

scmrepo/scmrepo/fs.py

Line 175 in 28973b1

tree_obj = scm.pygit2.get_tree_obj(rev=resolved)

which is against the general scmrepo behavior, where it should go through the backends.

Also GitFileSystem defaulting to pygit2 by itself is also pretty odd by itself.