rust-revlog: add methods for getting parent revs and entries Differential Revision: https://phab.mercurial-scm.org/D12442

rust-changelog: start parsing changeset data This patch makes `ChangelogRevisionData` do some coarse, line-level splitting of the changeset data into manifest node, user, timestamp, files list, and description. There are no (in-tree) users of these functions yet, but I've added tests to prevent regressions. We'll surely add callers at some point. Differential Revision: https://phab.mercurial-scm.org/D12439

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