manifest: use property instead of field for manifest revlog storage
The file caches we're using to avoid reloading the manifest from disk everytime
has an annoying bug that causes the in memory structure to not be reloaded if
the mtime and the size haven't changed. This causes a breakage in the tests
because the manifestlog is not being reloaded after a commit+strip operation in
mq (the mtime is the same because it all happens in the same second, and the
resulting size is the same because we add 1 and remove 1). The only reason this
doesn't affect the manifest itself is because we touch it so often that we
had already reloaded it after the commit, but before the strip.
Once the entire manifest has migrated to manifestlog, we can get rid of these
properties, since then the manifestlog will be touched after the commit, but
before the strip, as well.