midx: add MIDX_PROGRESS flag

midx: add MIDX_PROGRESS flag

Add the MIDX_PROGRESS flag and update the
write|verify|expire|repack functions in midx.h
to accept a flags parameter.  The MIDX_PROGRESS
flag indicates whether the caller of the function
would like progress information to be displayed.
This patch only changes the method prototypes
and does not change the functionality. The
functionality change will be handled by a later patch.

Signed-off-by: William Baker <William.Baker@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>