pack-revindex: introduce a new API

pack-revindex: introduce a new API

In the next several patches, we will prepare for loading a reverse index
either in memory (mapping the inverse of the .idx's contents in-core),
or directly from a yet-to-be-introduced on-disk format. To prepare for
that, we'll introduce an API that avoids the caller explicitly indexing
the revindex pointer in the packed_git structure.

There are four ways to interact with the reverse index. Accordingly,
four functions will be exported from 'pack-revindex.h' by the time that
the existing API is removed. A caller may:

 1. Load the pack's reverse index. This involves opening up the index,
    generating an array, and then sorting it. Since opening the index
    can fail, this function ('load_pack_revindex()') returns an int.
    Accordingly, it takes only a single argument: the 'struct
    packed_git' the caller wants to build a reverse index for.

    This function is well-suited for both the current and new API.
    Callers will have to continue to open the reverse index explicitly,
    but this function will eventually learn how to detect and load a
    reverse index from the on-disk format, if one exists. Otherwise, it
    will fallback to generating one in memory from scratch.

 2. Convert a pack position into an offset. This operation is now
    called `pack_pos_to_offset()`. It takes a pack and a position, and
    returns the corresponding off_t.

    Any error simply calls BUG(), since the callers are not well-suited
    to handle a failure and keep going.

 3. Convert a pack position into an index position. Same as above; this
    takes a pack and a position, and returns a uint32_t. This operation
    is known as `pack_pos_to_index()`. The same thinking about error
    conditions applies here as well.

 4. Find the pack position for a given offset. This operation is now
    known as `offset_to_pack_pos()`. It takes a pack, an offset, and a
    pointer to a uint32_t where the position is written, if an object
    exists at that offset. Otherwise, -1 is returned to indicate
    failure.

    Unlike some of the callers that used to access '->offset' and '->nr'
    directly, the error checking around this call is somewhat more
    robust. This is important since callers should always pass an offset
    which points at the boundary of two objects. The API, unlike direct
    access, enforces that that is the case.

    This will become important in a subsequent patch where a caller
    which does not but could check the return value treats the signed
    `-1` from `find_revindex_position()` as an index into the 'revindex'
    array.

Two design warts are carried over into the new API:

  - Asking for the index position of an out-of-bounds object will result
    in a BUG() (since no such object exists), but asking for the offset
    of the non-existent object at the end of the pack returns the total
    size of the pack.

    This makes it convenient for callers who always want to take the
    difference of two adjacent object's offsets (to compute the on-disk
    size) but don't want to worry about boundaries at the end of the
    pack.

  - offset_to_pack_pos() lazily loads the reverse index, but
    pack_pos_to_index() doesn't (callers of the former are well-suited
    to handle errors, but callers of the latter are not).

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