The no_symbol_match makes command line completion a lot less useful. It
disables any matches for file names with symbols in them. This prevents
completing things like ":b foo/bar" to ":b foo/bar.txt" or ":b foo_" to
":b baz/foo_bar.txt". Add an option `disallow_symbol_nonprefix_matching`
that prevents a match if it contains a symbol and isn't a prefix match.
Make that option the default. Add the option to documentation and tests.
Add to the examples for command line setup disabling the option.
6460f979b9