commit: use ui.configbool when checking 'commitsubrepos' setting on --amend

Before this fix, having

   [ui]
   commitsubrepos = False

in the config file lead to

   $ hg ci --amend -mx
   abort: cannot amend recursively

mercurial/commands.py

@@ -1228,7 +1228,7 b' def commit(ui, repo, *pats, **opts):'
     bheads = repo.branchheads(branch)
 
     if opts.get('amend'):
-        if ui.config('ui', 'commitsubrepos'):
+        if ui.configbool('ui', 'commitsubrepos'):
             raise util.Abort(_('cannot amend recursively'))
 
         old = repo['.']