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.
abort: Failed to parse .../t/.hg/hgrc
File contains no section headers.
file: .../t/.hg/hgrc, line: 1
'invalid\n'