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
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls -A
.hg
a
unknown