filter: Make `--exclude "dir/"` equivalent to `--exclude "dir/**"`

Rclone uses directory exclusions to cut down the listing it has to do,
so before this fix `--exclude dir/` would make sure nothing in `dir/`
was scanned, **except** if --fast-list was used, in which case only
the directory was excluded and everything within it was included.

This is rather unexpected, so this patch makes `--exclude dir/` be
equivalent to `--exclude dir/**`, meaning that excluding a directory
excludes it and its contents.

We can't do the same for --include without changing the semantics of
filtering slightly.

Fixes #3375

fs/filter/filter.go

@@ -267,6 +267,10 @@ func (f *Filter) addDirGlobs(Include bool, glob string) error {
 func (f *Filter) Add(Include bool, glob string) error {
 	isDirRule := strings.HasSuffix(glob, "/")
 	isFileRule := !isDirRule
+	// Make excluding "dir/" equivalent to excluding "dir/**"
+	if isDirRule && !Include {
+		glob += "**"
+	}
 	if strings.Contains(glob, "**") {
 		isDirRule, isFileRule = true, true
 	}

fs/filter/filter_test.go

@@ -523,6 +523,21 @@ func TestFilterAddDirRuleOrFileRule(t *testing.T) {
 + (^|/)potato$
 --- Directory filter rules ---
 + ^.*$`,
+		},
+		{
+			false,
+			"potato/",
+			`--- File filter rules ---
+- (^|/)potato/.*$
+--- Directory filter rules ---
+- (^|/)potato/.*$`,
+		},
+		{
+			true,
+			"potato/",
+			`--- File filter rules ---
+--- Directory filter rules ---
++ (^|/)potato/$`,
 		},
 		{
 			false,