[MERGE] Micro-tweaks to sha routines.

[Andrew Bennetts]

Robert Collins wrote:
> This just tweaks the sha helpers in osutils to make less calls - they
> are often called from inner loops. 

bb:tweak

> -def sha_strings(strings):
> +def sha_strings(strings, factory=sha.new):
>      """Return the sha-1 of concatenation of strings"""
> -    s = sha.new()
> +    s = factory()
>      map(s.update, strings)
>      return s.hexdigest()

Please rename 'factory' to '_factory'.  It's an optimisation hack, not an
argument that callers should use.

Otherwise this all looks sensible to me.

-Andrew.