dirstate: make DirstateItem constructor accept fallback value
This could arguably goes in the previous changeset, but I wanted to keep that
previous changeset small to focus more on the user code and the documentation.
Differential Revision: https://phab.mercurial-scm.org/D11687
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true