cmd/go/internal/vcweb: simplify hgHandler cancellation

cmd/go/internal/vcweb: simplify hgHandler cancellation

This uses the new Cancel and WaitDelay fields of os/exec.Cmd
(added in #50436) to interrupt or kill the 'hg serve' command
when its incoming http.Request is canceled.

This should keep the vcweb hg handler from getting stuck if 'hg serve'
hangs after the request either completes or is canceled.

Fixes #57597 (maybe).

Change-Id: I53cf58e8ab953fd48c0c37f596f99e885a036d9b
Reviewed-on: https://go-review.googlesource.com/c/go/+/460997
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>