csum-file.c: use unsafe SHA-1 implementation when available

csum-file.c: use unsafe SHA-1 implementation when available

Update hashwrite() and friends to use the unsafe_-variants of hashing
functions, calling for e.g., "the_hash_algo->unsafe_update_fn()" instead
of "the_hash_algo->update_fn()".

These callers only use the_hash_algo to produce a checksum, which we
depend on for data integrity, but not for cryptographic purposes, so
these callers are safe to use the unsafe (non-collision detecting) SHA-1
implementation.

To time this, I took a freshly packed copy of linux.git, and ran the
following with and without the OPENSSL_SHA1_UNSAFE=1 build-knob. Both
versions were compiled with -O3:

    $ git for-each-ref --format='%(objectname)' refs/heads refs/tags >in
    $ valgrind --tool=callgrind ~/src/git/git-pack-objects \
        --revs --stdout --all-progress --use-bitmap-index <in >/dev/null

Without OPENSSL_SHA1_UNSAFE=1 (that is, using the collision-detecting
SHA-1 implementation for both cryptographic and non-cryptographic
purposes), we spend a significant amount of our instruction count in
hashwrite():

    $ callgrind_annotate --inclusive=yes | grep hashwrite | head -n1
    159,998,868,413 (79.42%)  /home/ttaylorr/src/git/csum-file.c:hashwrite [/home/ttaylorr/src/git/git-pack-objects]

, and the resulting "clone" takes 19.219 seconds of wall clock time,
18.94 seconds of user time and 0.28 seconds of system time.

Compiling with OPENSSL_SHA1_UNSAFE=1, we spend ~60% fewer instructions
in hashwrite():

    $ callgrind_annotate --inclusive=yes | grep hashwrite | head -n1
     59,164,001,176 (58.79%)  /home/ttaylorr/src/git/csum-file.c:hashwrite [/home/ttaylorr/src/git/git-pack-objects]

, and generate the resulting "clone" much faster, in only 11.597 seconds
of wall time, 11.37 seconds of user time, and 0.23 seconds of system
time, for a ~40% speed-up.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>