Problem: When a match ends in part of concealed text highlighting, it might
mess up concealing by resetting prev_syntax_id.
Solution: Do not reset prev_syntax_id and add a test to verify. (Christian
Brabandt, closes#1092)
Problem: syn-cchar defined with matchadd() does not appear if there are no
other syntax definitions which matches buffer text.
Solution: Check for startcol. (Ozaki Kiichi, haya14busa, closes#757)
