codereview: force hg update after hg pull -u during hg sync

codereview: force hg update after hg pull -u during hg sync

If you hg update your client to an earlier CL, then
hg sync will move you back to tip if it pulls anything in,
but it will leave you where you are if it doesn't pull anything in.
That's confusing: make hg sync always update to tip.

R=golang-dev, bradfitz, r, dsymonds
CC=golang-dev
https://golang.org/cl/10456044