logging: add new --no-ignore-messages flag

The new --no-ignore-messages flag permits suppressing errors related to
parsing .gitignore or .ignore files. These error messages can be somewhat
annoying since they can surface from repositories that one has no control
over.

Fixes #646

src/args.rs

@@ -63,6 +63,7 @@ pub struct Args {
     maxdepth: Option<usize>,
     mmap: bool,
     no_ignore: bool,
+    no_ignore_messages: bool,
     no_ignore_parent: bool,
     no_ignore_vcs: bool,
     no_messages: bool,
@@ -308,6 +309,12 @@ impl Args {
         self.no_messages
     }
 
+    /// Returns true if error messages associated with parsing .ignore or
+    /// .gitignore files should be suppressed.
+    pub fn no_ignore_messages(&self) -> bool {
+        self.no_ignore_messages
+    }
+
     /// Create a new recursive directory iterator over the paths in argv.
     pub fn walker(&self) -> ignore::Walk {
         self.walker_builder().build()
@@ -327,7 +334,7 @@ impl Args {
         }
         for path in &self.ignore_files {
             if let Some(err) = wd.add_ignore(path) {
-                if !self.no_messages {
+                if !self.no_messages && !self.no_ignore_messages {
                     eprintln!("{}", err);
                 }
             }
@@ -402,6 +409,7 @@ impl<'a> ArgMatches<'a> {
             maxdepth: self.usize_of("maxdepth")?,
             mmap: mmap,
             no_ignore: self.no_ignore(),
+            no_ignore_messages: self.is_present("no-ignore-messages"),
             no_ignore_parent: self.no_ignore_parent(),
             no_ignore_vcs: self.no_ignore_vcs(),
             no_messages: self.is_present("no-messages"),