Add structure representing hash algorithm

Since in the future we want to support an additional hash algorithm, add
a structure that represents a hash algorithm and all the data that must
go along with it.  Add a constant to allow easy enumeration of hash
algorithms.  Implement function typedefs to create an abstract API that
can be used by any hash algorithm, and wrappers for the existing SHA1
functions that conform to this API.

Expose a value for hex size as well as binary size.  While one will
always be twice the other, the two values are both used extremely
commonly throughout the codebase and providing both leads to improved
readability.

Don't include an entry in the hash algorithm structure for the null
object ID.  As this value is all zeros, any suitably sized all-zero
object ID can be used, and there's no need to store a given one on a
per-hash basis.

The current hash function transition plan envisions a time when we will
accept input from the user that might be in SHA-1 or in the NewHash
format.  Since we cannot know which the user has provided, add a
constant representing the unknown algorithm to allow us to indicate that
we must look the correct value up.  Provide dummy API functions that die
in this case.

Finally, include git-compat-util.h in hash.h so that the required types
are available.  This aids people using automated tools their editors.

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

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