rust-changelog: added a test for `NULL_REVISION` special case The result is due to `Revlog.get_rev_data()` returning an empty byte string for `NULL_REVISION`, followed by special case for emtpty byte strings in `ChangelogRevisionData::new()`.

rust-changelog: made doc-comments more consistent The most important is the one about `data_for_rev`, that looked like a copy-paste leftover (got me confused first time I read this code, before I actually learned there were both `Entry` and RevisionData`. In the comment for the `struct`, "changelog" was probably more about the format in general (as documented elsewhere) than as an identifier. Some of the "Return something" had "of", half had "for".

extras: re-use Projection from jaraco.collections