hex: retire get_sha1_hex()

hex: retire get_sha1_hex()

The naming convention around get_sha1_hex() and its friends is
awkward these days, after "struct object_id" was introduced.

There are three public functions around this area:

 * get_sha1_hex()       - use the implied the_hash_algo, fill uchar *
 * get_oid_hex()        - use the implied the_hash_algo, fill oid *
 * get_oid_hex_algop()  - use the passed algop, fill oid *

Between the latter two, the "_algop" suffix signals whether the
the_hash_algo is used as the implied algorithm or the caller should
pass an algorithm explicitly.  That is very much understandable and
is a good convention.

Between the former two, however, the "SHA1" vs "OID" in the names
differentiate in what type of variable the result is stored.

We could argue that it makes sense to use "SHA1" to mean "flat byte
buffer" to honor the historical practice in the days before "struct
object_id" was invented, but the natural fourth friend of the above
group would take an algop and fill a flat byte buffer, and it would
be strange to name it get_sha1_hex_algop().  Do we use the passed in
algo, or are we limited to SHA-1 ;-)?

In fact, such a function exists, albeit as a private helper function
used by the implementation of these functions, and is named a lot
more sensibly: get_hash_hex_algop().

Correct the misnomer of get_sha1_hex() and use "hash", instead of
"sha1", as "flat byte buffer that stores binary (as opposed to
hexadecimal) representation of the hash".

The four (2x2) friends now become:

 * get_hash_hex()       - use the implied the_hash_algo, fill uchar *
 * get_oid_hex()        - use the implied the_hash_algo, fill oid *
 * get_hash_hex_algop() - use the passed algop, fill uchar *
 * get_oid_hex_algop()  - use the passed algop, fill oid *

As there are only two remaining calls to get_sha1_hex() in the
codebase right now, the blast radious of this change is fairly
small.

Signed-off-by: Junio C Hamano <gitster@pobox.com>