This reverts commit 6d0c0ffe.

Reason for revert: this appears to  break builders if reference
is a commit hash:

git log --format=%B -n1 HEAD failed with code 128

Original change's description:
> Add previous_got_revision and previous_got_revision_cp to output
>
> As chrome browser infra works to improve CQ times, we would like to
> easily glance at builds and see how many new commits were checked out.
> Currently you have to open up bot_update stdout to see the previous
> HEAD revision and go from there. Having this conveniently outputted
> would be very convenient.
>
> If the bot has previously checked out the repo, output which revision
> and commit position were checked out.
>
> example LED build showing new output: https://ci.chromium.org/swarming/task/51592e46d6a70e10?server=chromium-swarm.appspot.com
>
> Change-Id: Ib8a6f152a593446ce4082477bc9ada44f1fd1fc3
> Bug: 1170568
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2644829
> Reviewed-by: Josip Sokcevic <sokcevic@google.com>
> Commit-Queue: Stephanie Kim <kimstephanie@google.com>

TBR=infra-scoped@luci-project-accounts.iam.gserviceaccount.com,sokcevic@google.com,kimstephanie@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1170568
Change-Id: I50d6ef6802c2eb1614cca804391c7826a79a2cf9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2657023Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>