diff view puts revision back into $REV

Ignore-this: c671d4887772c0e08b04687472a5142f

When a diff is shown without giving a revision (eg when coming from recent
changes), the diff is done agains the latest non-current revisions. This
revision is now put back into the global $REV.

This fixes a problem with the restore button not visible.

However this only works when the restore button is called after the diff
view. A better solution probably needs to be found.

darcs-hash:20090921213302-7ad00-aa9ef53f9bc9ea8d0dc51e55b049943feb11c412.gz