ewah: `bitmap_equals_ewah()`

ewah: `bitmap_equals_ewah()`

Prepare to reuse existing pseudo-merge bitmaps by implementing a
`bitmap_equals_ewah()` helper.

This helper will be used to see if a raw bitmap (containing the set of
parents for some pseudo-merge) is equal to any existing pseudo-merge's
commits bitmap (which are stored as EWAH-compressed bitmaps on disk).

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