Problem:
Normally, `:drop +41 foo.txt` will open foo.txt with the cursor on line
41. But if foo.txt is already open, it instead is a no-op, even if the
cursor is on a different line.
Steps to reproduce:
nvim --clean foo.txt
:drop +30 foo.txt
Solution:
Handle +cmd in ex_drop().
(cherry picked from commit 3647b821ea)
837faf7065