midx.c: respect 'pack.writeBitmapHashcache' when writing bitmaps

midx.c: respect 'pack.writeBitmapHashcache' when writing bitmaps

In the previous commit, the bitmap writing code learned to propagate
values from an existing hash-cache extension into the bitmap that it is
writing.

Now that that functionality exists, let's expose it by teaching the 'git
multi-pack-index' builtin to respect the `pack.writeBitmapHashCache`
option so that the hash-cache may be written at all.

Two minor points worth noting here:

  - The 'git multi-pack-index write' sub-command didn't previously read
    any configuration (instead this is handled in the base command). A
    separate handler is added here to respect this write-specific
    config option.

  - I briefly considered adding a 'bitmap_flags' field to the static
    options struct, but decided against it since it would require
    plumbing through a new parameter to the write_midx_file() function.

    Instead, a new MIDX-specific flag is added, which is translated to
    the corresponding bitmap one.

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