Update to handle updated git references api

[bitwiseman]

Version 2.18 and older use this:
https://docs.github.com/en/enterprise/2.18/user/rest/reference/git#get-all-references

Version 2.19 and new use this:
https://docs.github.com/en/rest/reference/git#references

Specifically, getting references is now done using git/matching-refs instead of git/refs.

See #822, #842, #844, #845. Also see #852 for likely feature that needs to be added to make this safe.

That said 2.18 will be unsupported after August 2020. Perhaps the right behavior is to try default to the new API and have a switch to enable the old one.