hash: add a constant for the default hash algorithm

hash: add a constant for the default hash algorithm

Right now, SHA-1 is the default hash algorithm in Git.  However, this
may change in the future.

We have many places in our code that use the SHA-1 constant to indicate
the default hash if none is specified, but it will end up being more
practical to specify this explicitly and clearly using a constant for
whatever the default hash algorithm is.  Then, if we decide to change it
in the future, we can simply replace the constant representing the
default with a new value.

For these reasons, introduce GIT_HASH_DEFAULT to represent the default
hash algorithm.

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