Improve IterableList Type

[Byron]

Adjust git.utils.IterableList to be more convenient.

Implement the 'contains' method to allow something like:
"upsteam" in repo.heads

[migurski]

Ultra-belated 👍 to this, running into bugs like:

>>> 'foo' in r.remotes
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "…/site-packages/git/util.py", line 593, in __contains__
    rval = list.__contains__(self, attr)
  File "…/site-packages/git/remote.py", line 405, in __eq__
    return self.name == other.name
AttributeError: 'str' object has no attribute 'name'

[Byron]

It seems this one has been fixed quite a while ago.