checkpatch: SPDX integration breaks --root

[Charlemagne Lasse]

Hi,

I am now getting python errors when using --root with checkpatch.pl
and not being inside the linux repository.

$ ./linux-next/scripts/checkpatch.pl --root=/usr/src/linux-next
--strict -f linux-next/Makefile
FAIL: /usr/src
Traceback (most recent call last):
 File /usr/src/linux-next/scripts/spdxcheck.py", line 230, in <module>
   repo = git.Repo(os.getcwd())
 File "/usr/lib/python2.7/dist-packages/git/repo/base.py", line 156, in __init__
   raise InvalidGitRepositoryError(epath)
InvalidGitRepositoryError: /usr/src

total: 0 errors, 0 warnings, 0 checks, 1726 lines checked

linux-next/Makefile has no obvious style problems and is ready for submission.