builtin/repack.c: convert `--write-midx` to an `OPT_CALLBACK`

builtin/repack.c: convert `--write-midx` to an `OPT_CALLBACK`

Change the --write-midx (-m) flag from an OPT_BOOL to an OPT_CALLBACK
that accepts an optional mode argument. Introduce an enum with
REPACK_WRITE_MIDX_NONE and REPACK_WRITE_MIDX_DEFAULT to distinguish
between the two states, and update all existing boolean checks
accordingly.

For now, passing no argument (or just `-m`) selects the default mode,
preserving existing behavior. A subsequent commit will add a new mode
for writing incremental MIDXs.

Extract repack_write_midx() as a dispatcher that selects the
appropriate MIDX-writing implementation based on the mode.

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