ignore/gitignore: skip BOM at start of ignore file

This matches Git's behavior.

Fixes #2177, Closes #2782

crates/ignore/tests/gitignore_skip_bom.gitignore

@@ -0,0 +1,2 @@
+ignore/this/path
+# This file begins with a BOM (U+FEFF)

crates/ignore/tests/gitignore_skip_bom.rs

@@ -0,0 +1,17 @@
+use ignore::gitignore::GitignoreBuilder;
+
+const IGNORE_FILE: &'static str = "tests/gitignore_skip_bom.gitignore";
+
+/// Skip a Byte-Order Mark (BOM) at the beginning of the file, matching Git's
+/// behavior.
+///
+/// Ref: <https://github.com/BurntSushi/ripgrep/issues/2177>
+#[test]
+fn gitignore_skip_bom() {
+    let mut builder = GitignoreBuilder::new("ROOT");
+    let error = builder.add(IGNORE_FILE);
+    assert!(error.is_none(), "failed to open gitignore file");
+    let g = builder.build().unwrap();
+
+    assert!(g.matched("ignore/this/path", false).is_ignore());
+}