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

rust-changelog: don't skip empty lines when iterating over changeset lines The first empty line in the changeset indicates the end of headers and beginning of description. Callers can't know figure out where that position is if empty lines are skipped. Differential Revision: https://phab.mercurial-scm.org/D12426

rust-requirements: allow loading repos with `bookmarksinstore` requirement `rhg` does support bookmarks, so it can load repos with the `bookmarksinstore` requirement just as well as other repos. Differential Revision: https://phab.mercurial-scm.org/D12425

rebase: while rewriting desc hashes, ignore ambiguous prefix "hashes" If a repo is sufficiently large, a six digit number "hash prefix" can somewhat easily reference an ambiguous hash prefix. Differential Revision: https://phab.mercurial-scm.org/D12552