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
https://bz.mercurial-scm.org/1089
$ hg init
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..