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
mkcommit() {
name="$1"
shift
echo "$name" > "$name"
hg add "$name"
hg ci -m "$name" "$@"
}