dirstate-item: add a `set_possibly_dirty` method
See inline documentation for details.
The pushes the AMBIGUOUS_TIME implementation further down the line within the
DirstateItem only. When this cleanup is done we will be able to stop using this
representation internally.
Differential Revision: https://phab.mercurial-scm.org/D11119
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors