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