rust-changelog: remove special parsing of empty changelog data for null rev
For the null revision, `Revlog::get_rev_data()` will return an empty
string (of bytes). We currently handle that case in
`ChangelogRevisionData::manifest_node()`. However, it's going to be
ugly to have special handling for the null revision for each future
method on `ChangelogRevisionData`. This patch therefore restructures
the code so we instead initialize the struct with valid data for the
null revision.
Differential Revision: https://phab.mercurial-scm.org/D12438
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.