overlayworkingctx: default branch to base context's branch

This matches what the dirstate does (it reuses working copy parent's
branch unless told otherwise). By moving the default out of
`rebase.commitmemorynode()`, it will let us clean that up better
later.

Differential Revision: https://phab.mercurial-scm.org/D7821

hgext/rebase.py

@@ -1430,7 +1430,7 b' def commitmemorynode(repo, p1, p2, wctx,'
 
     # By convention, ``extra['branch']`` (set by extrafn) clobbers
     # ``branch`` (used when passing ``--keepbranches``).
-    branch = repo[p1].branch()
+    branch = None
     if b'branch' in extra:
         branch = extra[b'branch']
 

mercurial/context.py

@@ -2445,6 +2445,9 b' class overlayworkingctx(committablectx):'
                 # necessary for memctx to register a deletion.
                 return None
 
+        if branch is None:
+            branch = self._wrappedctx.branch()
+
         return memctx(
             self._repo,
             parents,