Add implied-branches pragma directive

When two projects have dissimilar stable branch names, for example,
stable/pike and stable/jewel, but should generally be used together
and therefore share job variants, it can be difficult to make that
happen.  Currently, one must add explicit multi-branch matchers
to every such job and project-template.

This allows a user to add a pragma directive to indicate all the
jobs in a file should apply to multiple branches.

Change-Id: I57cf159992d8f501cbaf41aef19562951ef6b7ea
12 files changed