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
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.