[MERGE] don't use gzip -9 to compress knits.

[Robert Collins]

I tracked down the root cause of the performance difference in GzipFile.

Its non-obvious unless you read the docstring, but the default
compression level is 9 - this is quite different to the command line
gzip and not something I'd thought to look for before.

Sigh.

Anyhow, heres an easy 30 second win on a large tree commit.

-Rob
-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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