i18n: omit redundant translatable synopsis text to avoid xgettext warning

This empty translatable synopsis text causes xgettext warning below:

    Empty msgid.  It is reserved by GNU gettext:
    gettext("") returns the header entry with
    meta information, not the empty string.

This "synopsis" argument of @command annotation is omitable.

mercurial/debugcommands.py

@@ -949,8 +949,7 b' def debugfileset(ui, repo, expr, **opts)'
         ui.write("%s\n" % f)
 
 @command('debugformat',
-         [] + cmdutil.formatteropts,
-        _(''))
+         [] + cmdutil.formatteropts)
 def debugformat(ui, repo, **opts):
     """display format information about the current repository