distros: add branch infrastructure

distros: add branch infrastructure

Since the distro nightlies are not version controlled we cannot use
the usual mechanisms for detecting regressions. Special case things
appropriately. We use an OLD_REVISION of "flight-NNN" to signify that
the old revision is another flight and not a tree revision.

A grep over $NEW_REVISION needed adjusting since NEW_REVISION is empty
in this mode, leading to "grep <filename>" which hangs waiting for
stdin.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>