transient--insert-suffix: Allow same key with different predicates

Previously it was not possible to add two bindings that use the
same key but different predicates using `transient-insert-suffix'.
Such only superficially conflicting bindings had to be added when
the prefix was defined using `transient-define-prefix'.

Now we use a heuristic to allow `transient-insert-suffix' to add
such bindings as well.

If both bindings define a predicate and these predicates are not
equal, then the two bindings are assumed to be distinct.  If on
the other hand at least one of the bindings lacks a predicate or
the predicates are equal, then the new binding is considered a
replacement for the old binding, as before.

Closes #106.

lisp/transient.el

@@ -1065,7 +1065,15 @@ example, sets a variable use `transient-define-infix' instead.
               ;; We must keep `mem' until after we have inserted
               ;; behind it, which `transient-remove-suffix' does
               ;; not allow us to do.
-              (setq action 'replace)
+              (let ((spred (transient--suffix-predicate suf))
+                    (epred (transient--suffix-predicate elt)))
+                ;; If both suffixes have a predicate and they
+                ;; are not identical, then the probability is
+                ;; high that we want to keep both.
+                (when (or (not spred)
+                          (not epred)
+                          (equal spred epred))
+                  (setq action 'replace)))
             (transient-remove-suffix prefix key))))
       (cl-ecase action
         (insert  (setcdr mem (cons elt (cdr mem)))
@@ -1801,6 +1809,20 @@ value.  Otherwise return CHILDREN as is."
                           (apply #'derived-mode-p if-not-derived))))
    (t default)))
 
+(defun transient--suffix-predicate (spec)
+  (let ((plist (nth 2 spec)))
+    (seq-some (lambda (prop)
+                (when-let ((pred (plist-get plist prop)))
+                  (list prop pred)))
+              '( :if :if-not
+                 :if-nil :if-non-nil
+                 :if-mode :if-not-mode
+                 :if-derived :if-not-derived
+                 :inapt-if :inapt-if-not
+                 :inapt-if-nil :inapt-if-non-nil
+                 :inapt-if-mode :inapt-if-not-mode
+                 :inapt-if-derived :inapt-if-not-derived))))
+
 ;;; Flow-Control
 
 (defun transient--init-transient ()