Support case-insensitive ignores (fixes #1511).

internal/fnmatch/fnmatch.go

@@ -38,6 +38,13 @@ func Convert(pattern string, flags int) (*regexp.Regexp, error) {
 		}
 	}
 
+	// Support case insensitive ignores
+	ignore := strings.TrimPrefix(pattern, "(?i)")
+	if ignore != pattern {
+		flags |= FNM_CASEFOLD
+		pattern = ignore
+	}
+
 	if flags&FNM_NOESCAPE != 0 {
 		pattern = strings.Replace(pattern, "\\", "\\\\", -1)
 	} else {

internal/fnmatch/fnmatch_test.go

@@ -53,6 +53,8 @@ var testcases = []testcase{
 	{"**/foo.txt", "bar/baz/foo.txt", FNM_PATHNAME, true},
 
 	{"foo.txt", "foo.TXT", FNM_CASEFOLD, true},
+	{"foo.txt", "foo.TXT", 0, false},
+	{"(?i)foo.txt", "foo.TXT", 0, true},
 
 	// These characters are literals in glob, but not in regexp.
 	{"hey$hello", "hey$hello", 0, true},