add percent escaping for string.gsub (#1991)
add percent escaping for string.gsub (#1991)

In `string.gsub()`, the `%` character has special meaning and must be escaped to be treated verbatim, otherwise the "invalid use of '%' in replacement string" warning will show up.

This adds a verbatim() function for that purpose. It fixes this warning for situations where `'%` characters are in the current path (cwd), version control branch names, or in the previous `PROMPT` set by the user.
cmderdev/cmder,cmderdev/cmder