libcmd/repl: do not clear the environment on empty reloads

If `:reload` is used with no file loaded, the environment is completely
cleared.

That's a surprising behavior and one of the root cause behind #50.

Environment clear is only performed if there's files to reload.

Change-Id: Icb014f490a83aaa4d758dbf37822db16d5bb65c1
Signed-off-by: Raito Bezarius <raito@lix.systems>

lix/libcmd/repl.cc

@@ -1354,6 +1354,11 @@ void NixRepl::initEnv()
 
 void NixRepl::reloadFiles()
 {
+    if (loaded.empty() && getValues().empty()) {
+        notice("No file to reload, skipping");
+        return;
+    }
+
     initEnv();
 
     loadFiles();