Even when "git pull --rebase=preserve" (and the underlying "git
rebase --preserve") can complete without creating any new commit
(i.e. fast-forwards), it still insisted on having a usable ident
information (read: user.email is set correctly), which was less
than nice.  As the underlying commands used inside "git rebase"
would fail with a more meaningful error message and advice text
when the bogus ident matters, this extra check was removed.

* jk/rebase-i-drop-ident-check:
  rebase-interactive: drop early check for valid ident