Use OpenSSL's SHA-1 routines rather than builtin block-sha1 routines.
This improves performance on SHA1 operations on Intel processors.

OpenSSL 1.0.2 has made considerable performance improvements and
support the Intel hardware acceleration features.  See:
https://software.intel.com/en-us/articles/improving-openssl-performance
https://software.intel.com/en-us/articles/intel-sha-extensions

To test this I added/staged a single file in a gigantic
repository having a 450MB index file.  The code in read-cache.c
verifies the header SHA as it reads the index and computes a new
header SHA as it writes out the new index.  Therefore, in this test
the SHA code must process 900MB of data.  Testing was done on an
Intel I7-4770 CPU @ 3.40GHz (Intel64, Family 6, Model 60) CPU.

The block-sha1 version averaged 5.27 seconds.
The OpenSSL    version averaged 4.50 seconds.

================================================================

$ echo xxx >> project.mk
$ time /e/blk_sha/bin/git.exe add project.mk

real    0m5.207s
user    0m0.000s
sys     0m0.250s

$ echo xxx >> project.mk
$ time /e/blk_sha/bin/git.exe add project.mk

real    0m5.362s
user    0m0.015s
sys     0m0.234s

$ echo xxx >> project.mk
$ time /e/blk_sha/bin/git.exe add project.mk

real    0m5.300s
user    0m0.016s
sys     0m0.250s

$ echo xxx >> project.mk
$ time /e/blk_sha/bin/git.exe add project.mk

real    0m5.216s
user    0m0.000s
sys     0m0.250s

================================================================
$ echo xxx >> project.mk
$ time /e/openssl/bin/git.exe add project.mk

real    0m4.431s
user    0m0.000s
sys     0m0.250s

$ echo xxx >> project.mk
$ time /e/openssl/bin/git.exe add project.mk

real    0m4.478s
user    0m0.000s
sys     0m0.265s

$ echo xxx >> project.mk
$ time /e/openssl/bin/git.exe add project.mk

real    0m4.690s
user    0m0.000s
sys     0m0.250s

$ echo xxx >> project.mk
$ time /e/openssl/bin/git.exe add project.mk

real    0m4.420s
user    0m0.000s
sys     0m0.234s

================================================================
Signed-off-by: NJeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>