branch: do not attempt to track HEAD implicitly

branch: do not attempt to track HEAD implicitly

With branch.autosetupmerge=always, 'HEAD' becomes a valid target for
branch tracking.  This is a bit silly, and in fact the code already
guards against it when attempting it explicitly:

  $ git checkout HEAD^0
  $ git branch -t foo
  fatal: Cannot setup tracking information; starting point is not a branch.

Silently drop the HEAD candidate in the implicit (i.e. without -t
flag) case, so that the branch starts out without an upstream.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>