manifest: use 't' for tree manifest flag
We currently use 'd' to indicate that a manifest entry is a
directory. Let's switch to 't', since that's not a valid hex digit and
therefore easier to spot in the raw manifest data.
This will break any existing repos with tree manifests, but it's still
an experimental feature and there are probably only a few test repos
in existence with 'd' flags.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.