Re: [PATCH 09/16] documentation: add documentation for the bitmap format

Pinning the bitmap index on the reverse index adds complexity (lookups
are two-step: first find the entry in the reverse index, and then find
the SHA1 in the index) and is measurably slower, in both loading and
lookup times. Since Git doesn't have a memory problem, it's very hard
to make an argument for design that is more complex and runs slower to
save memory.

Sorting by SHA1 will generate a random distribution. This will require
you to inflate the entire bitmap on every fetch request, in order to
do the "contains" operation.  Sorting by pack offset allows us to
inflate only the bits we need as we are walking the graph, since they
are usually at the start of the bitmap.

What is the general size in bytes of the SHA1 sorted bitmaps?  If they
are much larger, the size of the bitmap has an impact on how fast you
can perform bitwise operations on them, which is important for fetch
when doing wants AND NOT haves.