argv: tweak the meaning of zero

This commit makes a small tweak to the --max-columns flag. Namely, if
the value of the flag is 0, then ripgrep behaves as-if the flag were
absent.

This is useful in the context of ripgrep reading configuration from the
environment. For example, an end user might set --max-columns=150, but we
should permit the user to disable this setting when needed. Using -M0 is
a nice way to do that.

We do this because a zero value for --max-columns isn't particularly
meaningful. We do leave the --max-count, --max-filesize and --maxdepth
flags alone though, since a zero value for those flags is potentially
meaningful. (--max-count even has tests for ripgrep's behavior when
given a value of 0.)

src/app.rs

@@ -1034,6 +1034,8 @@ fn flag_max_columns(args: &mut Vec<RGArg>) {
     const LONG: &str = long!("\
 Don't print lines longer than this limit in bytes. Longer lines are omitted,
 and only the number of matches in that line is printed.
+
+When this flag is omitted or is set to 0, then it has no effect.
 ");
     let arg = RGArg::flag("max-columns", "NUM").short("M")
         .help(SHORT).long_help(LONG)