Support unprotected file patterns (#16395)

Fixes #16381

Note that changes to unprotected files via the web editor still cannot be pushed directly to the protected branch. I could easily add such support for edits and deletes if needed. But for adding, uploading or renaming unprotected files, it is not trivial.

* Extract & Move GetAffectedFiles to modules/git

services/forms/repo_form.go

@@ -199,6 +199,7 @@ type ProtectBranchForm struct {
 	DismissStaleApprovals         bool
 	RequireSignedCommits          bool
 	ProtectedFilePatterns         string
+	UnprotectedFilePatterns       string
 }
 
 // Validate validates the fields