[gocui] Truncate line after eraseInLineFromCursor

We still need to truncate the line after executing the eraseInLineFromCursor
instruction, to remove left-over content from a previous rendering; just not at
the current write position, but after the spaces that were inserted.

Without this fix you would sometimes see left-over junk when scrolling
left/right using the H/L keys in the commits panel.

Author: stefanhaller