lib/sha1: use the git implementation of SHA-1

lib/sha1: use the git implementation of SHA-1

For ChromiumOS, we use SHA-1 to verify the integrity of the root
filesystem.  The speed of the kernel sha-1 implementation has a major
impact on our boot performance.

To improve boot performance, we investigated using the heavily optimized
sha-1 implementation used in git.  With the git sha-1 implementation, we
see a 11.7% improvement in boot time.

10 reboots, remove slowest/fastest.

Before:

  Mean: 6.58 seconds Stdev: 0.14

After (with git sha-1, this patch):

  Mean: 5.89 seconds Stdev: 0.07

The other cool thing about the git SHA-1 implementation is that it only
needs 64 bytes of stack for the workspace while the original kernel
implementation needed 320 bytes.

Signed-off-by: Mandeep Singh Baines <[email protected]>
Cc: Ramsay Jones <[email protected]>
Cc: Nicolas Pitre <[email protected]>
Cc: Herbert Xu <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: [email protected]
Signed-off-by: Linus Torvalds <[email protected]>