This is a reland of 856ccef0

Use a tempfile instead of stdout for gclient revinfo output

Original change's description:
> bot_update: Don't use gclient sync output and rely on gclient revinfo.
>
> In preparation for skipping gclient sync if there are no DEPS changes.
>
> Bug: 1199853
> Change-Id: Ib9b4ab803bc574a384c661765cee5e4c1de5baae
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2912259
> Reviewed-by: Anthony Polito <apolito@google.com>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

Bug: 1199853
Change-Id: Ib50c4e943cbb51e4a2463beee9b7d1dee824ad29
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2910950Reviewed-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>