This reverts commit 72a245e4.

Reason for revert: crbug.com/1022617

Original change's description:
> Fetch the input commit ref when both ref and id are specified.
> 
> This causes --refs refs/head/master to be added to bot_update calls when
> the input build message contains a commit id and the refs/head/master
> ref. This is an effective no-op.
> 
> When bot_update clones a git repository, it results in HEAD referring to
> master in contrast to the non-clone case which results in a detached
> HEAD. This prevents fetching refs/heads/master, so detach the HEAD after
> cloning.
> 
> Bug: 922150
> Change-Id: I357a78305020218e9f5d350832a706b8f2b27967
> Recipe-Nontrivial-Roll: build
> Recipe-Nontrivial-Roll: build_limited_scripts_slave
> Recipe-Nontrivial-Roll: infra
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1900735
> Commit-Queue: Garrett Beaty <gbeaty@chromium.org>
> Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>

TBR=ehmaldonado@chromium.org,gbeaty@chromium.org

Change-Id: Ibd6c6ed272b0c98f82be50cefa521aaee0444f11
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 922150, 1022617
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1904099Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>