dirstate-item: add a `from_v1_data` constructor
This class method is dedicated to building a DirstateItem from the data
available in the "dirstate-v1" format. Since that format is frozen, this
constructor will never change (unlike the `__init__` one).
Differential Revision: https://phab.mercurial-scm.org/D11118
test-convert-git.t
test-subrepo-git.t
test-patchbomb-tls.t