overlaycontext: allow calling copydata() on clean context

We should just report no copy if the context is clean.

Differential Revision: https://phab.mercurial-scm.org/D6358

mercurial/context.py

@@ -1913,7 +1913,7 b' class overlayworkingctx(committablectx):'
         if self.isdirty(path):
             return self._cache[path]['copied']
         else:
-            raise error.ProgrammingError('copydata() called on clean context')
+            return None
 
     def flags(self, path):
         if self.isdirty(path):

tests/test-rebase-inmemory.t

@@ -774,8 +774,14 b' Test rebasing a commit with copy informa'
   $ hg co -q 0
   $ hg mv a b
   $ hg ci -qm 'rename a to b'
-  $ hg rebase -d 1 2>&1 | grep '** ProgrammingError'
-  ** ProgrammingError: copydata() called on clean context
+  $ hg rebase -d 1
+  rebasing 2:b977edf6f839 "rename a to b" (tip)
+  merging a and b to b
+  saved backup bundle to $TESTTMP/rebase-rename/.hg/strip-backup/b977edf6f839-0864f570-rebase.hg
+  $ hg st --copies --change .
+  A b
+    a
+  R a
   $ cd ..
 
 Test rebasing when the file we are merging in destination is empty