Since `rcirc-markup-text-functions' is meant to apply only to the text,
the changed code, which applied it to the entirety of the message was
bugged.  The simplest example is `\me NICK' with `rcirc-color', where
NICK is supposed to be colored but isn't because the message reads
`[... NICK]' and the `]' is recognized as a part of the nick.

The patch should fix the issue, although it is possible that some
already existing functions put to `rcirc-markup-text-functions' will
break due to them expecting the previous behavior.  I still propose the
patch is applied as the issue is a bug on those functions' end and not
rcirc.