rebase: give a better error message for bogus branch

rebase: give a better error message for bogus branch

When you give a non-existent branch to git-rebase, it spits
out the usage. This can be confusing, since you may
understand the usage just fine, but simply have made a
mistake in the branch name.

Before:

  $ git rebase origin bogus
  Usage: git rebase ...

After:

  $ git rebase origin bogus
  fatal: no such branch: bogus
  Usage: git rebase ...

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>