Be better with short circuiting with --quiet.

It didn't make sense for --quiet to be part of the printer, because --quiet
doesn't just mean "don't print," it also means, "stop after the first
match is found." This needs to be wired all the way up through directory
traversal, and it also needs to cause all of the search workers to quit
as well. We do it with an atomic that is only checked with --quiet is
given.

Fixes #116.

src/printer.rs

@@ -33,8 +33,6 @@ pub struct Printer<W> {
     heading: bool,
     /// Whether to show every match on its own line.
     line_per_match: bool,
-    /// Whether to suppress all output.
-    quiet: bool,
     /// Whether to print NUL bytes after a file path instead of new lines
     /// or `:`.
     null: bool,
@@ -42,8 +40,7 @@ pub struct Printer<W> {
     replace: Option<Vec<u8>>,
     /// Whether to prefix each match with the corresponding file name.
     with_filename: bool,
-
-    /// The choice of Colours
+    /// The choice of colors.
     color_choice: ColorChoice
 }
 
@@ -54,7 +51,6 @@ struct ColorChoice {
 }
 
 impl ColorChoice {
-
     #[cfg(unix)]
     pub fn new() -> ColorChoice {
         ColorChoice {
@@ -86,7 +82,6 @@ impl<W: Terminal + Send> Printer<W> {
             file_separator: None,
             heading: false,
             line_per_match: false,
-            quiet: false,
             null: false,
             replace: None,
             with_filename: false,
@@ -141,12 +136,6 @@ impl<W: Terminal + Send> Printer<W> {
         self
     }
 
-    /// When set, all output is suppressed.
-    pub fn quiet(mut self, yes: bool) -> Printer<W> {
-        self.quiet = yes;
-        self
-    }
-
     /// Replace every match in each matching line with the replacement string
     /// given.
     ///
@@ -168,11 +157,6 @@ impl<W: Terminal + Send> Printer<W> {
         self.has_printed
     }
 
-    /// Returns true if the printer has been configured to be quiet.
-    pub fn is_quiet(&self) -> bool {
-        self.quiet
-    }
-
     /// Flushes the underlying writer and returns it.
     pub fn into_inner(mut self) -> W {
         let _ = self.wtr.flush();
@@ -222,9 +206,6 @@ impl<W: Terminal + Send> Printer<W> {
     /// Prints the context separator.
     pub fn context_separate(&mut self) {
         // N.B. We can't use `write` here because of borrowing restrictions.
-        if self.quiet {
-            return;
-        }
         if self.context_separator.is_empty() {
             return;
         }
@@ -398,9 +379,6 @@ impl<W: Terminal + Send> Printer<W> {
     }
 
     fn write(&mut self, buf: &[u8]) {
-        if self.quiet {
-            return;
-        }
         self.has_printed = true;
         let _ = self.wtr.write_all(buf);
     }
@@ -411,9 +389,6 @@ impl<W: Terminal + Send> Printer<W> {
     }
 
     fn write_file_sep(&mut self) {
-        if self.quiet {
-            return;
-        }
         if let Some(ref sep) = self.file_separator {
             self.has_printed = true;
             let _ = self.wtr.write_all(sep);