ewah: factor out bitmap growth

ewah: factor out bitmap growth

We auto-grow bitmaps when somebody asks to set a bit whose position is
outside of our currently allocated range. Other operations besides
single bit-setting might need to do this, too, so let's pull it into its
own function.

Note that we change the semantics a little: you now ask for the number
of words you'd like to have, not the id of the block you'd like to write
to.

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