pack-bitmap: simplify `reuse_partial_packfile_from_bitmap()` signature

The signature of `reuse_partial_packfile_from_bitmap()` currently takes in a bitmap, as well as three output parameters (filled through pointers, and passed as arguments), and also returns an integer result. The output parameters are filled out with: (a) the packfile used for pack-reuse, (b) the number of objects from that pack that we can reuse, and (c) a bitmap indicating which objects we can reuse. The return value is either -1 (when there are no objects to reuse), or 0 (when there is at least one object to reuse). Some of these parameters are redundant. Notably, we can infer from the bitmap how many objects are reused by calling bitmap_popcount(). And we can similar compute the return value based on that number as well. As such, clean up the signature of this function to drop the "*entries" parameter, as well as the int return value, since the single caller of this function can infer these values themself. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>