localrepo: have stream_in() not modify its argument

Localrepo's stream_in function modified its requirements argument. This is not
good practice because it may be unexpected behavior to the caller.

mercurial/localrepo.py

@@ -1827,8 +1827,8 b' class localrepository(object):'
             # new requirements = old non-format requirements +
             #                    new format-related
             # requirements from the streamed-in repository
-            requirements.update(self.requirements - self.supportedformats)
-            self.requirements = requirements
+            self.requirements = requirements | (
+                    self.requirements - self.supportedformats)
             self._applyopenerreqs()
             self._writerequirements()