Proposal of Git policy updates

• Allow pristine-lfs instead of pristine-tar. This would allow to store tarballs with precision pristine-tar cannot offer, especially e.g. with xz tarballs.
• Mention why pristine-lfs may be better in some cases.
• Make the preference for tarballs vs Git history less strict: using Git history should be okay in cases when the upstream history is not violently branched or when commits happen relatively rarely.