fix: Fix bundle size diffs

[Gudahtt]

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 commit 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

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

• I've followed MetaMask Contributor Docs and MetaMask Extension Coding Standards.
• I've completed the PR template to the best of my ability
• I’ve included tests if applicable
• I’ve documented my code using JSDoc format if applicable
• I’ve applied the right labels on the PR (see labeling guidelines). 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.

[Gudahtt]

Switched from ref to sha so that this is less dependent upon the branch having been fetched in the checkout step.

Currently it is fetched, as the checkout action will fetch all branches when you specify a fetch-depth of 0. But I wanted this to be resilient to that changing in the future.

[metamaskbot]

Builds ready [76abec7]

• builds: chrome, firefox
• builds (flask): chrome, firefox
• builds (MMI): chrome
• builds (test): chrome, firefox
• builds (test-flask): chrome, firefox
• build viz: Build System
• mv3: Bundle Size Stats
• mv2: E2e Actions Stats
• storybook: Storybook
• typescript migration: Dashboard
• all artifacts
• bundle viz:

  • background: 0, 1, 2, 3, 4, 5, 6, 7
  • common: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13
  • ui: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13
  • content-script: 0
  • offscreen: 0

Page Load Metrics (1776 ± 147 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	472	2094	1569	391	188
		domContentLoaded	1478	2720	1758	310	149
		load	1486	2726	1776	307	147
		domInteractive	25	223	50	43	21
		backgroundConnect	5	51	13	10	5
		firstReactRender	15	86	45	26	12
		getState	4	51	14	15	7
		initialActions	0	0	0	0	0
		loadScripts	1071	2163	1317	266	128
		setupStore	5	58	12	15	7
		uiStartup	1642	3190	2034	372	178

Bundle size diffs

• background: 0 Bytes (0.00%)
• ui: 0 Bytes (0.00%)
• common: 0 Bytes (0.00%)