Problem: With 'fo' set to "a2" inserting a space in the first column may
cause the cursor to jump to the previous line.
Solution: Handle the case when there is no comment leader properly. (Tor
Perkins) Also fix that cursor is in the wrong place when spaces
get replaced with a Tab.
Problem: Cursor position wrong when joining multiple lines and
'formatoptions' contains "a". (Moshe Kamensky)
Solution: Adjust cursor position for skipped indent. (Carlo Teubner)