Issue 3719: CG: Add comment about git-cl editor

git-cl fires either the editor specified by the
EDITOR environment variable or vi if EDITOR isn't defined.
Commit mentions this in the CG

Signed-off-by: David Kastrup <dak@gnu.org>
(cherry picked from commit 5155adb1fe43e2b166c3101ad48f05d502eef31a)