Other-revision-jobs: Provide other_revision_job_suffix
This is a string, a function of the job name, that identifies the
class of `other revisions'. It is empty for main-revision jobs
and currently there is only `<delimiter>prev' for build-*-prev.
We are going to use this in the bisector.
Reimplement main_revision_job_cond in terms of this. No functional
change, except that the SQL optimiser may have more work to do.
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>