There is no policy... however, if you are making changes to Moodle code then you should be using a version control system and it makes sense to use Git. But... Git is not the easiest thing to learn. It's design is inconsistent, the documentation is terrible and it's not too hard to get into a big mess. There is lots of info on the Internet though and you really need to do a bit of reading to understand its basic principals or you will be in endless confusion.
It would help a lot if you tell us what you have done and/or what you are trying to do? Did you make changes to core in one version of Moodle and are now trying to upgrade to a new version of Moodle and keep your changes? If so, which versions?
These things are not always simple and Git can't do magic for you. If there is a conflict it is there for a reason and you really need to go and look and fix the code yourself. Just forcing it to accept the newer version will not always result in code that actually works.
THIS (again) is why you don't modify core code