templater: drop support for old style keywords (API)
These changes originated from several commits over a period of time, so I'm
slightly unsure if this is correct. But the tests pass.
relnotes: mention removed support for mixed log graph lines
This adds release notes for
264a2cbb25d0 (graphmod: remove support for
graph lines mixing parent/grandparent styles (BC), 2018-10-16).
Differential Revision: https://phab.mercurial-scm.org/D6407