manifestcache: protect write with `wlock` instead of `lock`
The `wlock` is taken by both `update` and `commit` type operation. This would
help persisting the cache more aggressively.
An explicit test is introduced. However, we can already see the effect of this
change on earlier test output.
$ 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