builtin/branch.c: remove useless branch_get

builtin/branch.c: remove useless branch_get

branch_get sometimes returns current_branch, which can be NULL (e.g., if
you're on a detached HEAD). Try:

  $ git branch HEAD
  fatal: no such branch 'HEAD'

  $ git branch ''
  fatal: no such branch ''

However, it seems weird that we'd check those cases here (and provide
such lousy messages). And indeed, dropping that and letting us
eventually hit create_branch() gives a much better message:

  $ git branch HEAD
  fatal: 'HEAD' is not a valid branch name.

  $ git branch ''
  fatal: '' is not a valid branch name.

Signed-off-by: Tao Qingyun <taoqy@ls-a.me>
Signed-off-by: Junio C Hamano <gitster@pobox.com>