Resolve any lingering deltas, write out the index file and rename the
packfile.

This will allow us to index a packfile as soon as we receive it from
the network as well as storing it with its final name so we don't need
to pass temporary file names around.

The new indexer needs to be able to bypass any kind of buffering, as
it's trying to map data that it has just written to disk.

The cleanup needs to happen anyway, so set the error code and jump
there instead of copying the code.

Plug a few leaks

This is Git yo. You can fetch stuff from the history if you need it.

Basic branch management API

Fix compilation warning

Allow references to bear non-7bit-ascii names on Windows

Rewrite status using diff