mq: make qselect fail properly on an empty guard

hgext/mq.py

@@ -126,6 +126,8 b' class queue:'
                 self.series_guards.append(self.guard_re.findall(comment))
 
     def check_guard(self, guard):
+        if not guard:
+            return _('guard cannot be an empty string')
         bad_chars = '# \t\r\n\f'
         first = guard[0]
         for c in '-+':

tests/test-mq-guards

@@ -44,6 +44,8 b' echo % should push b.patch'
 hg qpush
 
 hg qpop
+echo % test selection of an empty guard
+hg qselect ""
 hg qselect a
 echo % should push a.patch
 hg qpush

tests/test-mq-guards.out

@@ -15,6 +15,8 b' a.patch: +a'
 applying b.patch
 Now at: b.patch
 Patch queue now empty
+% test selection of an empty guard
+abort: guard cannot be an empty string
 number of unguarded, unapplied patches has changed from 2 to 3
 % should push a.patch
 applying a.patch