ads-github-merge-all-upstreams: avoid using 'git branch' porcelain command

[salewski]

This issue pertains to the version of 'ads-github-merge-all-upstreams' distributed with
ads-github-tools-0.1.1.

There are at least three different places where the program invokes 'git branch'
directly, but that practice should be avoided because git-branch is considered to be a
"porcelain" command.

We should instead use the "plumbing" command 'git symbolic-ref'. E.g., some flavor of:

    $ git symbolic-ref --short -q HEAD

See also:

• Junio C Hamano's blog post "Checking the current branch programatically"
  https://git-blame.blogspot.com/2013/06/checking-current-branch-programatically.html
  2013-06-10