emacs: Use whitelist instead of blacklist for term escaping
Previously, the term escaper used a blacklist of characters that
needed escaping. This blacklist turned out to be somewhat incomplete;
for example, it did not contain non-whitespace ASCII control
characters or Unicode "fancy quotes", both of which do require the
term to be escaped.
Switch to a whitelist of characters that are definitely safe to leave
unquoted. This fixes the broken test introduced by the previous
patch.