[MERGE] Be more efficient with sha operations during commit

[Robert Collins]

This saves 10 seconds or so on an initial commit of a mozilla tree by
using sha_string, rather than sha_strings.

As part of it I've removed an unused function from the _KnitData class,
and removed code to cache during add operations on knits - we do not use
that capability anywhere, and its semantically transparent in anycase -
it will just read again if needed, but doing a check for it, when we
never use it, adds up in big trees.

-Rob
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