bb: warn people when there are unmatched globs
bb: warn people when there are unmatched globs

For convenience, it's nice to ignore globs that have no match (e.g.
when switching around between branches).

But it is awful if you end up with unmatched things and don't know it
(right Gan?)

For each unmatched glob, print a warning; if there are ZERO match
globs, print a warning. Sometimes we *want* zero matched globs,
so this is not an error.

Signed-off-by: Ronald G. Minnich <f569d0af9a3481c4007b95c7180d6c41009f2513@gmail.com>

u-root/u-root,hugelgupf/u-root,rminnich/u-root,rminnich/u-root,u-root/u-root,GanShun/u-root,hugelgupf/u-root,rminnich/u-root,hugelgupf/u-root,u-root/u-root,GanShun/u-root,rminnich/u-root,GanShun/u-root,hugelgupf/u-root