repo: add a `wcachevfs` to access the `.hg/wcache/` directory
This wvfs will allow us to migrate various cache to the new `wcache` directory.
Helping with cache issues with "share".
#require test-repo jshint hg10
$ . "$TESTDIR/helpers-testrepo.sh"
run jshint on all tracked files ending in .js except vendored dependencies
$ cd "`dirname "$TESTDIR"`"
$ testrepohg locate 'set:**.js' \
> 2>/dev/null \
> | xargs jshint