hbisect.py: don't rely on __del__ to write the current state.

This is yet another page of the "Thou shalt not do too much inside
__del__ methods" book, in the "demandload and __del__ don't go well
together" chapter.

The bisect extension is broken in 0.9.1:

$ hg bisect init
$ hg bisect bad
Fatal Python error: Interpreter not initialized (version mismatch?)
Aborted

(yes, I tripled checked my instalation to make sure the problem is not
there)

It's been broken since revision fe1689273f84 moved the import of the
binascii module into a demandload.

(In details: the first time that "hg bisect bad" (or good) is called,
there are still no revisions saved in .hg/bisect/*, so bisect.__init__
doesn't call hg.bin on anything.  So, when we reach __del__, the
binascii module still hasn't been imported and we get that "nice"
message above.)

hgext/hbisect.py

@@ -50,7 +50,7 b' class bisect(object):'
             if r:
                 self.badrev = hg.bin(r.pop(0))
 
-    def __del__(self):
+    def write(self):
         if not os.path.isdir(self.path):
             return
         f = self.opener(self.good_path, "w")
@@ -288,7 +288,10 b' for subcommands see "hg bisect help\\"'
     if len(args) > bisectcmdtable[cmd][1]:
         ui.warn(_("bisect: Too many arguments\n"))
         return help_()
-    return bisectcmdtable[cmd][0](*args)
+    try:
+        return bisectcmdtable[cmd][0](*args)
+    finally:
+        b.write()
 
 cmdtable = {
     "bisect": (bisect_run, [], _("hg bisect [help|init|reset|next|good|bad]")),