bug#20084: comint-highlight-prompt overrides ANSI colors in 24.4

[Glenn Morris]

IMO:

1) It's too late to fix this "properly" for 24.5.
2) The problem from 14744 (sometimes flickering on output)
is less of an issue than this issue (prompt not coloured)

so I suggest reverting the changes from 14744 in emacs-24, and
fixing it "properly" in master.

BUT personally I haven't even bothered to check whether the changes from
14744 do actually cause this issue, so someone else will have to do the
work.