Fixed libgit2 bug/deprecated

In libgit2 the "Credentials" property is deprecated, instead
CredentialsHelper should be used. In 0.19 there is a bug with the
Credentials property when cloning, using the "correct" way
(CredentialsHelper) fixes the bug:
https://github.com/libgit2/libgit2sharp/issues/834

In order to be consistent I changed also the fetch method.

Sorry for the missing tests, but I really don't know how to write a test for this?