Fix release merge script

[frangio]

In the case of conflicts when merging release-vX.Y with master, we are not doing our best to resolve them automatically.

This PR:

1. Adds || true to allow git merge to continue even with conflicts. They should be resolved by the following rm and git add commands. If they are not resolved by that, git commit will fail, we have to resolve manually.
2. Adds --diff-filter=D to git diff, this just makes sure we are only listing deleted files and not anything else that has changed.
3. Uses git ls-files --deleted to make sure we don't git add anything that we don't intend to.

2 and 3 are not strictly related to the original issue but are just making the script more robust.

[changeset-bot]

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Latest commit: 95ed609

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[ernestognw]

LGTM, I'd just add some more comments since these are the sort of things we forget after a while

[ernestognw]

LGTM, I'd just add some more comments since these are the sort of things we forget after a while