extracting rename information from mercurial (between repository)

[Till Varoquaux]

Not so long ago it was asked on this list how one could extract rename
information between different revisions of the same repository, the
proposed solution was to parse diff  git's output.
I am now a basic superset of this problem:
suppose you now have have to repositories (branching out from a common
ancestor), how do you extract this information (AFAIK diff does work
between repostiries)?
My stub of a solution is:
given to repository a and b and to revisions $arev and $brev (sha1),
copy b to c, push -f $arev from a to c and and diff -git -r $arev -r
$brev in c...
As far as I am concerned this looks really bad, does anyone have a
cleaner solution?

Cheers,
Till