amend: make a copy of "extra" to avoid mutating an input

I don't know of any problems this has caused, it just seems less
surprising.

Differential Revision: https://phab.mercurial-scm.org/D1575

mercurial/cmdutil.py

@@ -3137,6 +3137,8 def amend(ui, repo, old, extra, pats, op
         # base     o - first parent of the changeset to amend
         wctx = repo[None]
 
+        # Copy to avoid mutating input
+        extra = extra.copy()
         # Update extra dict from amended commit (e.g. to preserve graft
         # source)
         extra.update(old.extra())