rebase: fix in-memory rebasing of copy of empty file

Classic Python mistake of unintentionally treating None and empty
string the same.

Differential Revision: https://phab.mercurial-scm.org/D6570

mercurial/context.py

@@ -2159,7 +2159,9 class overlayworkingctx(committablectx):
         # the file is marked as existing.
         if exists and data is None:
             oldentry = self._cache.get(path) or {}
-            data = oldentry.get('data') or self._wrappedctx[path].data()
+            data = oldentry.get('data')
+            if data is None:
+                data = self._wrappedctx[path].data()
 
         self._cache[path] = {
             'exists': exists,

tests/test-rebase-inmemory.t

@@ -795,12 +795,10 Test rebasing a commit with copy informa
   $ hg co -q 0
   $ hg mv a b
   $ hg ci -qm 'rename a to b'
-BROKEN: shouldn't crash
   $ hg rebase -d 1
   rebasing 2:b977edf6f839 "rename a to b" (tip)
   merging a and b to b
-  abort: b@c71e275f666f: not found in manifest!
-  [255]
+  saved backup bundle to $TESTTMP/rebase-rename-empty/.hg/strip-backup/b977edf6f839-0864f570-rebase.hg
   $ hg st --copies --change .
   A b
     a