hash: create union for hash context allocation

In various parts of our code, we want to allocate a structure
representing the internal state of a hash algorithm.  The original
implementation of the hash algorithm abstraction assumed we would do
that using heap allocations, and added a context size element to struct
git_hash_algo.  However, most of the existing code uses stack
allocations and conversion would needlessly complicate various parts of
the code.  Add a union for the purpose of allocating hash contexts on
the stack and a typedef for ease of use.  Use this union for defining
the init, update, and final functions to avoid casts.  Remove the ctxsz
element for struct git_hash_algo, which is no longer very useful.

This does mean that stack allocations will grow slightly as additional
hash functions are added, but this should not be a significant problem,
since we don't allocate many hash contexts.  The improved usability and
benefits from avoiding dynamic allocation outweigh this small downside.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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