Forgot to mention, I have configured diff.git=True, so all my tests are
effectively of "diff --git".  Have not tested traditional diff at all.

I see the same thing as Benoit: at the head of default, "diff <file>" is
much faster than in 1.4, but still 2x slower than 1.3.

Bisecting again, it looks like it got faster on default with this change:

changeset:   9991:a7d11deb47dd
user:        Benoit Boissinot <benoit.boissinot@ens-lyon.org>
date:        Thu Dec 03 01:01:49 2009 +0100
summary:     revlog: add fast path to ancestor

But it's still ~2x slower than 1.3.