folder filter: only assume regex if filter is ~fmt

doc/aerc-config.5.scd

@@ -226,7 +226,7 @@ Note that many of these configuration options are written for you, such as
 
 *folders*
 	Specifies the comma separated list of folders to display in the sidebar.
-	Supports regex patterns.
+	Names prefixed with ~ are interpreted as regular expressions.
 
 	Default: all folders
 

widgets/dirlist.go

@@ -161,12 +161,17 @@ func (dirlist *DirectoryList) Prev() {
 }
 
 func folderMatches(folder string, pattern string) bool {
+	if len(pattern) == 0 {
+		return false
+	}
+	if pattern[0] == '~' {
 		r, err := regexp.Compile(pattern)
 		if err != nil {
 			return false
 		}
-
 		return r.Match([]byte(folder))
+	}
+	return pattern == folder
 }
 
 // filterDirsByFoldersConfig sets dirlist.dirs to the filtered subset of the