feat: compare changes in current with master branch to get selectors

[Skisocks]

Previously only the most recent commit was used to identify changes to a helmfile and identify the selectors to use. This caused issues where if a second commit was made altering a different helmfile then the first helmfile would be ignored. This is fixed by comparing changes to the helmfiles in the current branch with what is currently in the default branch.

The --allow-no-matching-release flag has also been added. This stops the templating from erroring in the case where a namespace is removed (selector applied to the helmfile flags no longer exists).

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[Skisocks]

/cc @msvticket

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[msvticket]

While this already is an improvement there is another case I'd like you to handle:
A change has been made to another helmfile the "master" branch. That is you should consider all files that differ between the PR branch and master.

We are really only interested in those changes that have not been regenerated, but it is quite tricky to find out.

So you could use this git log command:

git log $defaultBranch...HEAD --name-only --pretty=format:

And then there is another case I don't see is considered: that the script isn't run on a PR branch, but on master.