Earlier we were passing basename instead of the fullpath and then joining with
the tmproot. This is wrong because the user can choose rev1a as wdir and in
those cases, tmproot should not be joined with the path.
I am working on refactoring extdiff logic so that we can have options like `diff
--tool` and encountered this. Although this patch in itself makes no difference,
however in future when a new caller of _runperfilediff() will be added, it
will be useful to directly pass on the full paths instead.