Fix edge cases with vendoring

I encountered two edge cases that threw exceptions when running "berks vendor" with berks 3.2.1:
* the PATH provided to "berks vendor" is relative
* a cookbook being vendored contains read-only files (such as mycookbook/.git/objects/13/310f74cd58123013ffdf6bd616456793ad44fb)

I also think we should just ignore `.git` during the vendor instead of this check. We can pass `.git` to "exclude".