use (forward-line -1) instead of (previous-line)

previous-line is for interactive use and its behaviour depends on
configuration
use goto-char instead of beginning-of-buffer
do not use goto-line but forward-line instead

goto-line is for interactive use only
comment unused function go--common-prefix
use inhibit-read-only instead of toggle-read-only

toggle-read-only is for interactive use only
require diff-mode during compilation
also load diff-mode during compilation
Closes gh-5