lib/ignore: Centralize handling of temporary filenames (fixes #3899)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3901
LGTM: calmh, AudriusButkevicius

lib/ignore/ignore.go

@@ -249,6 +249,22 @@ func (m *Matcher) clean(d time.Duration) {
 	}
 }
 
+// ShouldIgnore returns true when a file is temporary, internal or ignored
+func (m *Matcher) ShouldIgnore(filename string) bool {
+	switch {
+	case IsTemporary(filename):
+		return true
+
+	case IsInternal(filename):
+		return true
+
+	case m.Match(filename).IsIgnored():
+		return true
+	}
+
+	return false
+}
+
 func hashPatterns(patterns []Pattern) string {
 	h := md5.New()
 	for _, pat := range patterns {