Refactor "dump" sub-command (#30240)

Major changes:

* Move some functions like "addReader" / "isSubDir" /
"addRecursiveExclude" to a separate package, and add tests
* Clarify the filename&dump type logic and add tests
* Clarify the logger behavior and remove FIXME comments

Co-authored-by: Giteabot <teabot@gitea.io>

modules/timeutil/timestamp.go

@@ -21,8 +21,9 @@ var (
 )
 
 // MockSet sets the time to a mocked time.Time
-func MockSet(now time.Time) {
+func MockSet(now time.Time) func() {
 	mockNow = now
+	return MockUnset
 }
 
 // MockUnset will unset the mocked time.Time