[PATCH 05 of 10] repair: obtain and validate requirements for upgraded repo

[timeless]

Gregory Szorc wrote:
> +                            'missing: %s') % ', '.join(sorted(missingreqs)))

I'd argue ', ' should be _(', '), although we probably don't manage
this well or consistently...

Also, is it too early to start talking about -T support? -- See `hg
debuginstall`...