[gocui] Fix a bug in the code that clears to the end of the line

This code assumed that the length of the current line is the same as the current
x write position, which isn't necessarily true if we are overwriting existing
lines.

This hasn't been a problem before
#3687 because we would only use the
erase-to-eol feature in cases where the length of the lines either didn't
change, or where we called v.clear before. As of that PR though, we can run into
it in lazygit for example when pressing "+" to expand the commits view and then
going back.

Author: stefanhaller