CLI: Fix keyfile from/to parameter collision in merge command

Rename 'k' to 'f' because 'k' is already used to specify the key for the target database of the merge

* Remove short -f option from keepassxc-cli.1
* Remove -f option from keepassxc-cli merge
* Add test cases covering cli options for merge
* Add functional test for merge with keys

tests/TestCli.h

@@ -66,6 +66,7 @@ private slots:
     void testList();
     void testLocate();
     void testMerge();
+    void testMergeWithKeys();
     void testMove();
     void testOpen();
     void testRemove();