[MERGE] Reduce overhead on commit by using knit specific API

[Robert Collins]

This branch improves commit performance by removing a check for parents
being present when adding texts. On 55K texts with a pack repo thats
about 10 seconds saved.

The check in question is only performed during 'add_lines', not
add_lines_with_ghosts, so I addressed this by changing CommitBuilder to
use add_lines_with_ghosts, using a subclass of that to preserve the api
used by weave repositories when adding texts.

At the same time I noted that we have a trivial compatibility hunk in
place in CommitBuilder which only affects subclasses of CommitBuilder
that require that the root entry *not* be supplied. The only plugins can
can be affected by this are foreign repository suppliers - bzr-svn,
bzr-hg etc - which also support commits, and AFAIK that means only
bzr-svn which subclasses RootCommitBuilder and is unaffected. So I
cleaned up the compatibility layer too to help keep the code simpler.

I think this is safe for 0.91.

Rob



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