builtin/mv: convert assert(3p) into `BUG()`

builtin/mv: convert assert(3p) into `BUG()`

The use of asserts is discouraged in our codebase because they lead to
different behaviour depending on how Git is built. When being unsure
enough whether a condition always holds so that one adds the assert,
then the assert should probably trigger regardless of how Git is being
built.

Drop the call to assert(3p) in git-mv(1) and instead use `BUG()`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>