Code review comment for lp:~jameinel/bzr/1.16-commit-fulltext

This patch builds on the previous one, and changes things to:
1) VF.add_text becomes VF._add_text
2) Simplifies the api a lot, since it is meant to be used by a limited crowd. So I got rid of the parent_texts parameter, etc. 'commit' never used it, and neither does CHK repositories. So it doesn't really make sense to carry it around as legacy.

This mostly is around making 'commit' hold less data in memory. I think it is right around 3x the size of a large file (depending on text/binary, long/short lines, no eol, etc.) We are still too high for memory on GC repos, but that is the delta index code, and my other patch handles that.

I haven't really performance tested this by itself, because not computing the delta index makes such a large impact.