This allows for easier testing. The component is still complex, so it
requires more refactoring into smaller parts. But I can now safely do
that since I have specified the expected behaviour with a test.
I already discovered a bug while writing the tests: I want to group by
date irrespective of the timestamp. Therefore I need to parse the date
from the ISO timestamp of a commit.
Signed-off-by: André Jaenisch <andre.jaenisch@posteo.de>
