bitmap: implement bitmap_is_subset()

bitmap: implement bitmap_is_subset()

The bitmap_is_subset() function checks if the 'self' bitmap contains any
bitmaps that are not on in the 'other' bitmap. Up until this patch, it
had a declaration, but no implementation or callers. A subsequent patch
will want this function, so implement it here.

Helped-by: Junio C Hamano <redacted>
Signed-off-by: Derrick Stolee <redacted>
Signed-off-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>