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/1350748Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Auto-Submit: Clemens Hammacher <clemensh@chromium.org>