rebase slow on large trees

[Martin Pool]

Andrew Stubbs writes in <http://lists.linaro.org/pipermail/linaro-toolchain/2011-May/001206.html> that rebasing on huge trees is slow:

"""Last time I used it, I took lp:~ams-codesourcery/gcc-linaro/cs-merge-20110413, and rebased it to lp:gcc-linaro. This took at least an hour to run, possibly 2-3 hours, I don't recall - I got bored and went to do something else.

Note that some of the revisions on cs-merge-20110413 have now been committed to lp:gcc-linaro, so to repeat the experiment you'll need to only rebase as far as r99496, I think. It's actually only 3 revisions difference, I think."""

This might be improved by John's 2.4 workingtree speed improvements, but there is probably still more we can do.