Failes for submodule

[white-gecko]

Currently the quit store doesn't work on repos with submodules.

It produces a stacktrace like:

2022-03-22 16:43:44,694 - quit.modules.endpoint - ERROR - 'NoneType' object has no attribute 'type'
Traceback (most recent call last):
  File "…/QuitStore/quit/web/modules/endpoint.py", line 155, in sparql
    graph, commitid = quit.instance(branch_or_ref)
  File "…/QuitStore/quit/core.py", line 196, in instance
    for blob in self.getFilesForCommit(commit):
  File "…/QuitStore/quit/core.py", line 400, in getFilesForCommit
    for entity in commit.node().entries(recursive=True):
  File "…/QuitStore/quit/git.py", line 608, in entries
    node = Node(
  File "…/QuitStore/quit/git.py", line 564, in __init__
    if self.obj.type == pygit2.GIT_OBJ_TREE:
AttributeError: 'NoneType' object has no attribute 'type'

Maybe fix by doing #235