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://codereview.appspot.com/10456044
cmd/ld: fix line numbers when using fieldtrack

USEFIELD is a special kind of NOP, so treat it like a NOP
when generating the pc-ln table.

There are more invasive fixes that could be applied here.
I am going for minimum number of lines changed.

The smallest test case we know of is five distinct Go files
in four packages, and the bug only happens with
GOEXPERIMENT=fieldtrack enabled, which we don't
normally build with, so the test would never run
meaningfully anyway.

Fixes issue 5762.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/10495044
cmd/gc: fix escape analysis ordering

Functions without bodies were excluded from the ordering logic,
because when I wrote the ordering logic there was no reason to
analyze them.

But then we added //go:noescape tags that need analysis, and we
didn't update the ordering logic.

So in the absence of good ordering, //go:noescape only worked
if it appeared before the use in the source code.

Fixes issue 5773.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/10570043
crypto/sha1: provide a top-level Sum function
Makes it easy to ask the simple question, what is the hash of this data?

R=golang-dev, rsc, bradfitz
CC=golang-dev
https://codereview.appspot.com/10571043