Force 'Diff URL' to be built using the actual ancestor

It would be nice for:

1. quick reviews via web
2. quick access to the actual changes via web, even (and especially!) after the changes have been merged and versioning history will evolve

to force people to declare the actual commit id in a compare with the Moodle git repo (github for convenience).
There is no need to keep the forked Moodle branches aligned for the comparison to be in place - it would be better for other contributor workflow reasons.

The only con I could see is some extra noise by email when the PR is rebased against the new Moodle release branch, which is IMHO a good noise given the pro (including the risk that the contributor will remove the branch from her/his repo in the near future).

Examples:

• MDL-62600: https://github.com/mickhawkins/moodle/compare/6e445671c1...MDL-62600-master
• MDL-63050: https://github.com/andrewnicols/moodle/compare/6e8235c7d3...MDL-63050-master
• MDL-61732: https://github.com/moodle/moodle/compare/6153be6850869cdc3a6ae925dcf6e688ac481333...Dagefoerde:MDL-61732-master
• MDL-58785: https://github.com/moodle/moodle/compare/master...danielneis:MDL-58785_master_neis
• MDL-62319: https://github.com/moodle/moodle/compare/master...tuanngocnguyen:MDL-62319-Master
• MDL-13745: https://github.com/vmdef/moodle/compare/MDL-13745-master~1...MDL-13745-master
• MDL-63128: https://github.com/moodle/moodle/compare/master...roperto:MDL-63128-36-redis-cluster-support