Fix completion bug for special fish characters.

Issue #1108: If there are special characters like '{' in the
completion suggestions, then we fail to parse it successfully
as we are passing an unescaped version of the character to
parser_t::eval_args(...).

This causes us to retun w/o completion suggestions.

This bug was discovered while implementing 'git stash' completion
as the suggestion contained strings like 'stash@\{0\}'.

Th fix is to properly escape the string before parsing it.