Fix a bug with handling --ignore-file.

Namely, passing a directory to --ignore-file caused ripgrep to allocate
memory without bound.

The issue was that I got a bit overzealous with partial error
reporting. Namely, when processing a gitignore file, we should try
to use every pattern even if some patterns are invalid globs (e.g.,
a**b). In the process, I applied the same logic to I/O errors. In this
case, it manifest by attempting to read lines from a directory, which
appears to yield Results forever, where each Result is an error of the
form "you can't read from a directory silly." Since I treated it as a
partial error, ripgrep was just spinning and accruing each error in
memory, which caused the OOM killer to kick in.

Fixes #228

ignore/src/walk.rs

@@ -451,7 +451,7 @@ impl WalkBuilder {
     pub fn add_ignore<P: AsRef<Path>>(&mut self, path: P) -> Option<Error> {
         let mut builder = GitignoreBuilder::new("");
         let mut errs = PartialErrorBuilder::default();
-        errs.maybe_push_ignore_io(builder.add(path));
+        errs.maybe_push(builder.add(path));
         match builder.build() {
             Ok(gi) => { self.ig_builder.add_ignore(gi); }
             Err(err) => { errs.push(err); }