"Parent check" improvements (support for backport branches + "recursive" check)

[thbar]

In this PR:

1. Make sure that backport PRs (such as Backport #1074 to v2.1-wip #1076) are not blocked by the parent check

• Before: only the target branch was compared to its parent (commits brought by the PR itself were not included in the comparison, causing backport branches to be blocked).
• After: commits brought in by the PR itself are included in the comparison.

2. Make sure that v3.0-wip is "recursively" up-to-date from v2.0

• Before: the test would be green if v3.0-wip is up-to-date from v2.1-wip, but v2.1-wip is not itself up-to-date from v2.0
• After: both parents are checked

Testing

The script was run locally against the real merge refs of #1076 (passes), #1068 and #981 (still blocked while v2.1-wip is behind, pass once it is synced).

The first real run will be #1076, after v2.0 is merged into its branch again.

[TuThoThai]

Thanks for this @thbar!

[thbar]

A good way to review is to go commit by commit:

• b5f8214?w=1 is easy to review "with whitespaces ignored" in diff
• d650c6a next