tests: use 'f' in test-merge-criss-cross.t to prepare for recursive dumping Prepare for adding a test case with files in a directory.

util: make sortdict.keys() return a copy dict.keys() is documented to return a copy, so it's surprising that sortdict.keys() did not. I noticed this because we have an extension that calls readlocaltags(). That method tries to remove any tags that point to non-existent revisions (most likely stripped). However, since it's unintentionally working on the instance it's modifying, it sometimes fails to remove tags when there are multiple bad tags in a row. This was not caught because localrepo.tags() does an additional layer of filtering. sortdict is also used in other places, but I have not checked whether its keys() and/or __delitem__() methods are used there.