[build] Fix gcc_update -r with mercurial

[Paolo Bonzini]

On 10/11/2010 09:00 PM, Rainer Orth wrote:
> When trying to update a mainline hg tree to an older revision to work
> around a bootstrap failure, I noticed that gcc_update -rREV with
> mercurial doesn't work.  Instead, one needs to separate the hg pull and
> the hg update -rREV, which is exactly what this patch does.
>
> Tested by running gcc_update with and without -r and observing that it
> works as expected.
>
> Ok for mainline?

Ok.

Paolo