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
% creating repo
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