b4: include change-id in cover template

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 <redacted>
Acked-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>