This reverts commit b99e61c5.

Reason for revert:
Doesn't work for refs/branch-heads/*

Original change's description:
> bot_update: Refactor branch to detach and checkout.
> 
> Refactor _git_checkout to use information from
> get_target_branch_and_revision to know what branch to checkout after
> cloning.
> 
> Bug: 1104182
> Change-Id: Ib3ba57ca0b6803f172b85121c2a4b123f17bfb8c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2303291
> Reviewed-by: Josip Sokcevic <sokcevic@google.com>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

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

Change-Id: If51423df99fd9c164f8e42e0220ee2d9bc2a39f6
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1104182
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2310293Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>