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
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[255]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate