SHA1: support reset() and reuse instances

SHA1: support reset() and reuse instances

Allow SHA1 instances to be reused to compute another hash value, and
resume caching them in ObjectInserter and PackParser.  This shaves a
small amount of running time off parsing git.git's pack file:

  before   after
  ------   ------
  25.25s   25.55s
  25.48s   25.06s
  25.26s   24.94s

Almost noise (small difference), but recycling the instances reduces
some stress on the memory allocator finding two 80 word message block
arrays needed for hashing and collision detection.

Change-Id: I4af88a720e81460293bc5c5d1d3db1a831e7e228