Bug: Not working in first commit

[tyapochkin]

If dstrace is initialized in empty repo, there is the error during first commit:

`Calling DSTrace pre-commit hook

DSTrace pre-commit started.
Traceback (most recent call last):
File "/media/konstantin/docs/anaconda3/bin/dstrace", line 8, in
sys.exit(main())
File "/media/konstantin/docs/anaconda3/lib/python3.6/site-packages/dstrace/dstrace.py", line 354, in main
fire.Fire(CLI)
File "/media/konstantin/docs/anaconda3/lib/python3.6/site-packages/fire/core.py", line 138, in Fire
component_trace = _Fire(component, args, parsed_flag_args, context, name)
File "/media/konstantin/docs/anaconda3/lib/python3.6/site-packages/fire/core.py", line 471, in _Fire
target=component.name)
File "/media/konstantin/docs/anaconda3/lib/python3.6/site-packages/fire/core.py", line 675, in _CallAndUpdateTrace
component = fn(*varargs, **kwargs)
File "/media/konstantin/docs/anaconda3/lib/python3.6/site-packages/dstrace/dstrace.py", line 315, in pre_commit
self.convert_staged_notebooks()
File "/media/konstantin/docs/anaconda3/lib/python3.6/site-packages/dstrace/dstrace.py", line 331, in convert_staged_notebooks
for f in gp.get_git_staged():
File "/media/konstantin/docs/anaconda3/lib/python3.6/site-packages/dstrace/dstrace.py", line 122, in get_git_staged
return [i.a_path for i in self.repo.index.diff('HEAD')]
File "/media/konstantin/docs/anaconda3/lib/python3.6/site-packages/git/index/util.py", line 76, in check_default_index
return func(self, *args, **kwargs)
File "/media/konstantin/docs/anaconda3/lib/python3.6/site-packages/git/index/base.py", line 1227, in diff
other = self.repo.rev_parse(other)
File "/media/konstantin/docs/anaconda3/lib/python3.6/site-packages/git/repo/fun.py", line 333, in rev_parse
obj = name_to_object(repo, rev)
File "/media/konstantin/docs/anaconda3/lib/python3.6/site-packages/git/repo/fun.py", line 146, in name_to_object
raise BadName(name)
gitdb.exc.BadName: Ref 'HEAD' did not resolve to an object
`