Default to subrepo option to use --no-verify

[soraxas]

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This closes #193, and potentially adresses #323

Since the clone / pull / ... command in git subrepo are operating on a different repo, it often doesn't make much sense to trigger the hook, e.g., pre-commit hooks in the main repo.

This PR defaults to use --no-verify flag on these related operation, and the behaviour can also be configured via

The .gitrepo file:

[subrepo]
    ...
    verify = true  # Enable hooks for this subrepo