tags: behave better if a tags cache entry is partially written
This is done by discarding any partial cache entry, instead of
filling the partial cache entry with 0xff before.
Differential Revision: https://phab.mercurial-scm.org/D8095
$ 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
unknown