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>

.b4-cover-template

@@ -8,4 +8,5 @@ ${diffstat}
 ${range_diff}
 ---
 base-commit: ${base_commit}
+change-id: ${change_id}
 ${prerequisites}