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

tests/tests.rs

@@ -893,6 +893,13 @@ clean!(regression_206, "test", ".", |wd: WorkDir, mut cmd: Command| {
     assert_eq!(lines, format!("{}:test\n", path("foo/bar.txt")));
 });
 
+// See: https://github.com/BurntSushi/ripgrep/issues/228
+clean!(regression_228, "test", ".", |wd: WorkDir, mut cmd: Command| {
+    wd.create_dir("foo");
+    cmd.arg("--ignore-file").arg("foo");
+    wd.assert_err(&mut cmd);
+});
+
 // See: https://github.com/BurntSushi/ripgrep/issues/20
 sherlock!(feature_20_no_filename, "Sherlock", ".",
 |wd: WorkDir, mut cmd: Command| {