git-diff-tree -M performance regression in 'next'

Hi,

I added some time logging code to git-merge-recursive to see exactly
what we spend all the time on in merges which involves many changes,
such as a merge of a slightly modified v2.6.12 and an unmodified
v2.6.15.

I turned out that the rename detection took almost 10 minutes on my
machine. More specifically,

   git-diff-tree -r -M --diff-filter=R v2.6.12 v2.6.14

takes ~9 minutes with the current 'next'.

With 65416758cd83772f2f3c69f1bd1f501608e64745, which uses the delta
code to compute the similarity measure, the above git-diff-tree
invocation takes 1.50 minutes.

- Fredrik