bashcompletion: show available command-line switches for aliases

When auto-completing hg commands, aliases are listed, but not the available
switches for an alias, because `HGPLAIN=1` filters these out. Add a
`HGPLAINEXCEPT=alias` exception to resolve this.

We make heavy use of aliases that drive hg log with custom revsets, sorting and
the -G switch, but want our users to be able to auto-complete any additional
command-line switches.

contrib/bash_completion

@@ -184,7 +184,7 b' shopt -s extglob'
             return
         fi
 
-        opts=$(_hg_cmd debugcomplete --options "$cmd")
+        opts=$(HGPLAINEXCEPT=alias _hg_cmd debugcomplete --options "$cmd")
 
         COMPREPLY=(${COMPREPLY[@]:-} $(compgen -W '$opts' -- "$cur"))
         _hg_fix_wordlist