appview/db: move timeline-commits logic into MakeTimeline
also fixes a subtle bug: when timeline commits are populated from the
punchcard, only the current year's commits are available, however the
timeline can span across two years (as it does today: Jan 2026, Dec
2025, Nov 2025 ...).
Signed-off-by: oppiliappan <me@oppi.li>
authored by
tangled.org
50158d86
94470cd6