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.
os = <unloaded module 'os'>
os.system = <built-in function system>
os = <module 'os' from '?'>
util = <unloaded module 'util'>
util.system = <function system at 0x?>
util = <module 'mercurial.util' from '?'>
util.system = <function system at 0x?>
fred = <unloaded module 're'>
re = <unloaded module 'sys'>
fred = <unloaded module 're'>
fred.sub = <function sub at 0x?>
fred = <proxied module 're'>
re = <unloaded module 'sys'>
re.stderr = <open file '<whatever>', mode 'w' at 0x?>
re = <proxied module 'sys'>
node = <module 'mercurial.node' from '?'>