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>