This reverts commit ba83229a.

Reason for revert: After mail discussion we came to the conclusion that the old behavior makes more sense.

Original change's description:
> Fix semantics of git new-branch --upstream
> 
> Currently, the "--upstream A" option for new-branch behaves totally
> different than "--upstream_current". While "--upstream A" checks out
> branch A and then creates a new branch which tracks A,
> "--upstream_current" creates a new branch for the current HEAD and sets
> the upstream to the previously checked out branch.
> 
> As the documentation does not mention that any of the options changes
> the currently-checked-out commit (HEAD), this CL changes the semantics
> of "git new-branch --upstream A B" to be identical to "git checkout -b B
> && git branch --set-upstream-to A".
> 
> It also slightly extends the documentation to mention that in any case
> the new branch is based on HEAD.
> 
> R=​iannucci@chromium.org
> 
> Change-Id: Ic335d2caf27cb6afca1b8bc5a008424c0e880fca
> Reviewed-on: https://chromium-review.googlesource.com/c/1350748
> Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Auto-Submit: Clemens Hammacher <clemensh@chromium.org>

TBR=iannucci@chromium.org,tandrii@chromium.org,clemensh@chromium.org

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

Change-Id: I7463935af172f0801c7da94d2de106a02fc4c42e
Reviewed-on: https://chromium-review.googlesource.com/c/1362972Reviewed-by: Sergiy Belozorov <sergiyb@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>