Fix copyline for tmux 2.4
Fix copyline for tmux 2.4

With the current implementation, if your shell is :

$ echo 'foo'

your cursor is at the end of the line, it will copy `$ echo 'foo'` on
tmux 2.4, and `echo 'foo'` on tmux 2.3 and below.

This normalizes the behavior on both versions so that only `echo 'foo'`
is copied

tmux-plugins/tmux-yank