kdb-mount: experimental strategy feature

"--strategy unchanged" as discussed #1306
seems to be buggy:
e.g. if plugins add plugin config (like dump)
     parent keys disappear and it seems like
     the config was changed (although it was not)

doc/help/elektra-merge-strategy.md

@@ -40,7 +40,9 @@ Currently the following strategies exist:
  * cut:
    Removes existing keys below the resultpath and replaces them with the merged keyset.
 
- * import:
+ * unchanged: (EXPERIMENTAL, only for kdb-mount)
+   Do not fail if the operation does not change anything.
+
+ * import: (DEPRECATED, avoid using it)
    Preserves existing keys in the resultpath if they do not exist in the merged keyset.
    If the key does exist in the merged keyset, it will be overwritten.
-   (avoid using it)

src/tools/kdb/mount.cpp

@@ -14,6 +14,8 @@
 #include <cmdline.hpp>
 #include <mount.hpp>
 
+#include <keysetio.hpp>
+
 #include <fstream>
 #include <iostream>
 #include <iterator>
@@ -101,7 +103,9 @@ void MountCommand::buildBackend (Cmdline const & cl)
 		throw invalid_argument (mp + " is not a valid mountpoint");
 	}
 
-	if (cl.force)
+	KeySet dupMountConf = mountConf.dup ();
+
+	if (cl.force || cl.strategy != "preserve")
 	{
 		Key cutKey (Backends::mountpointsPath, KEY_END);
 		cutKey.addBaseName (mpk.getName ());
@@ -168,6 +172,29 @@ void MountCommand::buildBackend (Cmdline const & cl)
 	// Call it a day
 	outputMissingRecommends (backend.resolveNeeds (cl.withRecommends));
 	backend.serialize (mountConf);
+
+	if (cl.strategy == "unchanged" && cl.debug)
+	{
+		cout << "The new configuration is:" << endl;
+		std::cout << mountConf;
+		std::cout << "------------------------" << std::endl;
+		cout << "The configuration originally was:" << endl;
+		std::cout << dupMountConf;
+	}
+
+	if (!cl.force && (cl.strategy == "unchanged" && mountConf != dupMountConf))
+	{
+		// throw error because it is not unchanged
+		try
+		{
+			backend.setMountpoint (mpk, dupMountConf);
+		}
+		catch (MountpointAlreadyInUseException const & e)
+		{
+			throw MountpointAlreadyInUseException (
+				std::string ("Requested unchanged mountpoint but mount would lead to changes\n") + e.what ());
+		}
+	}
 }
 
 void MountCommand::readPluginConfig (KeySet & pluginConfig)

src/tools/kdb/mount.hpp

@@ -35,7 +35,7 @@ class MountCommand : public MountBaseCommand
 
 	virtual std::string getShortOptions () override
 	{
-		return "dfqiR0123cW";
+		return "dfqisR0123cW";
 	}
 
 	virtual std::string getSynopsis () override