b4: include change-id in cover template

With b4 0.15.2, I hit a local failure after sending a series with the in-tree cover template. The generated sent/<change-id>-vN tag contained base-commit, but did not contain change-id, and later b4 commands failed when trying to read it: CRITICAL: Tag sent/... does not contain change-id info Looking at b4's source, the sent tag message is derived from the rendered cover letter. The same code later parses that tag and expects both base-commit and change-id to be present. The default b4 cover template has both trailers, but our in-tree template only has base-commit. Add the missing change-id trailer next to base-commit so sent tags produced from the project template remain readable by b4's reroll and comparison logic. Signed-off-by: Chen Linxuan <me@black-desk.cn> Acked-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>