Change get_repo behavior,

[lukaszb]

Now change repo raises an exception when more than one backedn is found on given path, well i have a proposal to change that behavior since someone would like to keep one repository in two or more scm-ms.

First proposal is not to raise an exception but warning, and return a list of found repositories for example
[MercurialRepository,GitRepository]

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Reference: https://bitbucket.org/marcinkuzminski/vcs/issues/39/