Create an enum that allows us to distinguish between different
hashing algorithms. This enum is embedded into each
`git_hash_ctx` and will instruct the code to which hashing
function the particular request shall be dispatched.

As we do not yet have multiple hashing algorithms, we simply
initialize the hash algorithm to always be SHA1. At a later
point, we will have to extend the `git_hash_init_ctx` function to
get as parameter which algorithm shall be used.