use correct path in rel-eng/packages if package reside in git-root

if package reside in subdirectory, then in metadata file is written
  1.0 foo/
but if the package reside in git-root, then in metadata file is written
  1.0 /
which is not relative path and if it is evaluated by some tool, it is evaluated incorrectly
This patch will change this singularity to:
  1.0 ./
add missing import of commands

addressing:
Traceback (most recent call last):
  File "/usr/bin/tito", line 23, in <module>
    CLI().main(sys.argv[1:])
  File "/usr/lib/python2.7/site-packages/tito/cli.py", line 94, in main
    return module.main(argv)
  File "/usr/lib/python2.7/site-packages/tito/cli.py", line 638, in main
    scratch=self.options.scratch)
  File "/usr/lib/python2.7/site-packages/tito/release.py", line 490, in release
    self._git_release()
  File "/usr/lib/python2.7/site-packages/tito/release.py", line 507, in _git_release
    self.builder.tgz()
  File "/usr/lib/python2.7/site-packages/tito/builder.py", line 784, in tgz
    self.patch_upstream()
  File "/usr/lib/python2.7/site-packages/tito/distributionbuilder.py", line 34, in patch_upstream
    (status, output) = commands.getstatusoutput(
NameError: global name 'commands' is not defined
use correct path in rel-eng/packages if package reside in git-root for DistributionBuilder

if package is directly in git-root, then
os.path.join('foo.git', '/') will return '/', which is not what we want'
Merge pull request #80 from xsuchy/pull-req-missing-import

add missing import of commands
Merge pull request #78 from xsuchy/pull-req-relative-dir

use correct path in rel-eng/packages if package reside in git-root