bugfix: get git branch failed when only 'git fetch' is provided

bugfix: get git branch failed when only 'git fetch' is provided

File "/usr/local/lib/python2.7/dist-packages/functest/cli/commands/cli_env.py", line 58, in show
    branch = repo.head.reference
  File "/usr/local/lib/python2.7/dist-packages/git/refs/symbolic.py", line 258, in _get_reference
    raise TypeError("%s is a detached symbolic reference as it points to %r" % (self, sha))
TypeError: HEAD is a detached symbolic reference as it points to '2632ba86dd920267455ee10154b9e2ce4695a889'

JIRA: FUNCTEST-673

Change-Id: I2306dde254710d7d0e8a3529a19e3d8fdb6e5b02
Signed-off-by: SerenaFeng <feng.xiaowei@zte.com.cn>