mq: new commands qselect, qguard
implement quilt-style guards for mq.
guards allow to control whether patch can be pushed.
if guard X is active and patch is guarded by +X (called "posative guard"),
patch can be pushed. if patch is guarded by -X (called "nagative guard"),
patch cannot be pushed and is skipped.
use qguard to set/list guards on patches. use qselect to set/list
active guards.
also "qseries -v" prints guarded patches with "G" now.
port:4:export
port:4:vaportight
port:4:import/export
FIXME: history is wrong here
port:1:1:-:eggs:import
port:1:2:+:eggs:vaportight
port:1:3:+:eggs:import/export
port:0:2:+:spam:export
port:0:1:+:spam:import
port:4:import/export