MFH: -r403912

backout commit r403914, the quarterly branch has 2.0, not 2.1.

Fix a regression in 2.1 where tmux was misshandling key strokes if you
started typing too fast after a tmux command.

PR: 205372
Submitted by: me jahn de
Obtained from: https://github.com/tmux/tmux/commit/8c8cddbe022af5ece9949804cacffbee6b8972fa
Sponsored by: Absolight