fast-rebase: change assert() to BUG()

fast-rebase: change assert() to BUG()

assert() can succinctly document expectations for the code, and do so in
a way that may be useful to future folks trying to refactor the code and
change basic assumptions; it allows them to more quickly find some
places where their violations of previous assumptions trips things up.

Unfortunately, assert() can surround a function call with important
side-effects, which is a huge mistake since some users will compile with
assertions disabled.  I've had to debug such mistakes before in other
codebases, so I should know better.  Luckily, this was only in test
code, but it's still very embarrassing.  Change an assert() to an if
(...) BUG (...).

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>