midx: clear auxiliary .rev after replacing the MIDX

midx: clear auxiliary .rev after replacing the MIDX

When writing a new multi-pack index, write_midx_internal() attempts to
clean up any auxiliary files (currently just the MIDX's `.rev` file, but
soon to include a `.bitmap`, too) corresponding to the MIDX it's
replacing.

This step should happen after the new MIDX is written into place, since
doing so beforehand means that the old MIDX could be read without its
corresponding .rev file.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>