A replacement SHA routine that's slightly larger, but over twice as
fast. It's also faster and much smaller than the cryptolib version.

             size      speed    buffer size
original:    350B      2.3us     320B
cryptolib:  5776B      1.2us      80B
this code:   466B      1.0us     320B
alternate:  2112B      1.0us      80B
Signed-off-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>