release notes script does a bad job of weeding out fixes that have already landed in previous branches

[ckerr]

The release notes script generates a lot of entries in the 7.0.0 notes that should be omitted because they include fixes that were already included in previous releases.

For example, #19941 was included. Its description reads:

Backport of #19912

See that PR for details.

Notes: Fixed command-line scheme arguments from spilling over into each other.

And #19912 has a merged/6-0-x label. The 6-0-x backport was PR #19940 which was merged in 6b77e2e, so this should not have been included in the 7.0.0 release notes.

Invocation was

$ ./release/notes/index.js origin/6-0-x..origin/7-0-x --version v7.0.0 --explicit-links

[nornagon]

I think we do a lot better on this now!