github-ci: fix commit message for markdown

Found that commit message may consists of special characters which can
be used be Markdown as commands, like '`' or '```'. To avoid of it these
characters must be changed to some predefined names like for:

'\' - BACKSLASH
'`' - BACKTICK

Also added filter block to avoid of other not known symbols which we
could miss. This block converts commit message to HTML and then takes
only text from it.