tests/test-gendoc-el.t

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

#require docutils gettext

  $ $TESTDIR/check-gendoc el
  checking for parse errors