Accept process-filter t in Tramp

* lisp/net/tramp.el (tramp-handle-make-process):
* lisp/net/tramp-adb.el (tramp-adb-handle-make-process):
* lisp/net/tramp-sh.el (tramp-sh-handle-make-process): Filter can be t.

* test/lisp/net/tramp-tests.el (tramp-test29-start-file-process)
(tramp-test30-make-process): Test filter equal t.

lisp/net/tramp-adb.el

@@ -947,7 +947,7 @@ implementation will be used."
 	    (setq connection-type 'pty))
 	  (unless (memq connection-type '(nil pipe pty))
 	    (signal 'wrong-type-argument (list #'symbolp connection-type)))
-	  (unless (or (null filter) (functionp filter))
+	  (unless (or (null filter) (eq filter t) (functionp filter))
 	    (signal 'wrong-type-argument (list #'functionp filter)))
 	  (unless (or (null sentinel) (functionp sentinel))
 	    (signal 'wrong-type-argument (list #'functionp sentinel)))

lisp/net/tramp-sh.el

@@ -2785,7 +2785,7 @@ implementation will be used."
 	    (setq connection-type 'pty))
 	  (unless (memq connection-type '(nil pipe pty))
 	    (signal 'wrong-type-argument (list #'symbolp connection-type)))
-	  (unless (or (null filter) (functionp filter))
+	  (unless (or (null filter) (eq filter t) (functionp filter))
 	    (signal 'wrong-type-argument (list #'functionp filter)))
 	  (unless (or (null sentinel) (functionp sentinel))
 	    (signal 'wrong-type-argument (list #'functionp sentinel)))

lisp/net/tramp.el

@@ -4141,7 +4141,7 @@ substitution.  SPEC-LIST is a list of char/value pairs used for
 	  (setq connection-type 'pty))
 	(unless (memq connection-type '(nil pipe pty))
 	  (signal 'wrong-type-argument (list #'symbolp connection-type)))
-	(unless (or (null filter) (functionp filter))
+	(unless (or (null filter) (eq filter t) (functionp filter))
 	  (signal 'wrong-type-argument (list #'functionp filter)))
 	(unless (or (null sentinel) (functionp sentinel))
 	  (signal 'wrong-type-argument (list #'functionp sentinel)))

test/lisp/net/tramp-tests.el

@@ -4562,6 +4562,24 @@ This tests also `make-symbolic-link', `file-truename' and `add-name-to-file'."
 	;; Cleanup.
 	(ignore-errors (delete-process proc)))
 
+      (unwind-protect
+	  (with-temp-buffer
+	    (setq proc (start-file-process "test3" (current-buffer) "cat"))
+	    (should (processp proc))
+	    (should (equal (process-status proc) 'run))
+	    (set-process-filter proc t)
+	    (process-send-string proc "foo\n")
+	    (process-send-eof proc)
+	    ;; Read output.
+	    (with-timeout (10 (tramp--test-timeout-handler))
+	      (while (process-live-p proc)
+		(while (accept-process-output proc 0 nil t))))
+	    ;; No output due to process filter.
+	    (should (= (point-min) (point-max))))
+
+	;; Cleanup.
+	(ignore-errors (delete-process proc)))
+
       ;; Process connection type.
       (when (and (tramp--test-sh-p)
 		 (not (tramp-direct-async-process-p))
@@ -4735,6 +4753,28 @@ If UNSTABLE is non-nil, the test is tagged as `:unstable'."
 	;; Cleanup.
 	(ignore-errors (delete-process proc)))
 
+      (unwind-protect
+	  (with-temp-buffer
+	    (setq proc
+		  (with-no-warnings
+		    (make-process
+		     :name "test3" :buffer (current-buffer) :command '("cat")
+		     :filter t
+		     :file-handler t)))
+	    (should (processp proc))
+	    (should (equal (process-status proc) 'run))
+	    (process-send-string proc "foo\n")
+	    (process-send-eof proc)
+	    ;; Read output.
+	    (with-timeout (10 (tramp--test-timeout-handler))
+	      (while (process-live-p proc)
+		(while (accept-process-output proc 0 nil t))))
+	    ;; No output due to process filter.
+	    (should (= (point-min) (point-max))))
+
+	;; Cleanup.
+	(ignore-errors (delete-process proc)))
+
       ;; Process sentinel.
       (unwind-protect
 	  (with-temp-buffer