Fix bundle size diffs that are included in `metamaskbot` comments and
tracked historically in https://github.com/MetaMask/extension_bundlesize_stats

The diffs were incorrect because the "merge base" commit was wrong. The
merge base commit is the commit we're comparing the branch against. We
were calculating this using `git merge-base origin/main HEAD`, which
locally does give the correct answer. But in GitHub Actions, HEAD was
not set to the tip of the branch. Instead it was a new comment
generated during the checkout step, representing the target branch
merged into the current branch.

This resulted in the "merge base" always being the tip of `main` (at
the time of this workflow run). This in turn resulted in the "bundle
diff" including changes that were already made on `main`

Fixes #29858.

Fix bundle size diffs that are included in `metamaskbot` comments and
tracked historically in https://github.com/MetaMask/extension_bundlesize_stats

The diffs were incorrect because the "merge base" commit was wrong. The
merge base commit is the commit we're comparing the branch against. We
were calculating this using `git merge-base origin/main HEAD`, which
locally does give the correct answer. But in GitHub Actions, HEAD was
not set to the tip of the branch. Instead it was a new comment
generated during the checkout step, representing the target branch
merged into the current branch.

This resulted in the "merge base" always being the tip of `main` (at
the time of this workflow run). This in turn resulted in the "bundle
diff" including changes that were already made on `main`

Fixes #29858.

## **Description**

Fix bundle size diffs that are included in `metamaskbot` comments.

The diffs were incorrect because the "merge base" commit was wrong. The
merge base commit is the commit we're comparing the branch against. We
were calculating this using `git merge-base origin/main HEAD`, which
locally does give the correct answer. But in GitHub Actions, HEAD was
not set to the tip of the branch. Instead it was a new comment generated
during the checkout step, representing the target branch merged into the
current branch.

This resulted in the "merge base" always being the tip of `main` (at the
time of this workflow run). This in turn resulted in the "bundle diff"
including changes that were already made on `main`

[![Open in GitHub
Codespaces](https://github.com/codespaces/badge.svg)](https://codespaces.new/MetaMask/metamask-extension/pull/29862?quickstart=1)

## **Related issues**

Fixes #29858.

## **Manual testing steps**

I created a second draft PR to add diagnostic logs for testing this:
#29859

You can see an example of these logs here:
https://github.com/MetaMask/metamask-extension/actions/runs/12917863547/job/36025202434?pr=29859

Note that the `HEAD` resolves to a totally different commit than the
real head of the branch (which is accessible as `pull_request.head.sha`,
resulting in a different merge base.

## **Screenshots/Recordings**

N/A

## **Pre-merge author checklist**

- [x] I've followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask
Extension Coding
Standards](https://github.com/MetaMask/metamask-extension/blob/main/.github/guidelines/CODING_GUIDELINES.md).
- [x] I've completed the PR template to the best of my ability
- [x] I’ve included tests if applicable
- [x] I’ve documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [x] I’ve applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-extension/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

## **Pre-merge reviewer checklist**

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.