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>