ewah/bitmap: introduce bitmap_word_alloc()

ewah/bitmap: introduce bitmap_word_alloc()

In a following commit we will need to allocate a variable
number of bitmap words, instead of always 32, so let's add
bitmap_word_alloc() for this purpose.

Note that we have to adjust the block growth in bitmap_set(),
since a caller could now use an initial size of "0" (we don't
plan to do that, but it doesn't hurt to be defensive).

Helped-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>