run git from proper directory

lib/spack/spack/cmd/checkout.py

@@ -55,10 +55,6 @@ def known_commit_or_tag(ref):
 
 
 def checkout(parser, args):
-    deployment_required_args = {'remote': 'origin'}
-    deployment.setup_deployment_args('checkout', args,
-                                     deployment_required_args)
-
     remote = args.remote or 'origin'
     url = args.url
     ref = args.ref
@@ -66,6 +62,7 @@ def checkout(parser, args):
     global git  # make git available to called methods
     git = which('git', required=True)
 
+    with working_dir(spack.paths.prefix):
         # Always fetch branches
         branches = map(lambda b: b.strip('* '),
                        git('branch', output=str, error=str).split('\n'))