Merge branch 'js/rebase-i-allow-drop-on-a-merge'

During interactive rebase, using 'drop' on a merge commit lead to
an error, which was incorrect.

* js/rebase-i-allow-drop-on-a-merge:
  rebase -i: permit 'drop' of a merge commit

diff --cc sequencer.c
Simple merge

diff --cc t/t3404-rebase-interactive.sh
Simple merge