ci(l1,l2): lines of code report

[gianbelinche]

Motivation
The lines of code CI compares directly with main, so if the branch is not updated it gives fake results

Description
Now the CI compares to the most common commit between the branches

How to Test

This PR is not up to date with main, without the changes on the CI we would see a code report with the new changes on main.
Since this PR only changes the CI there should be no code changes on the report, but in order to show that it works, a commit was added where one line changed, a report was generated, and then the commit was reverted.
The old report is still present since the new run of the CI detected that there are no changes, so the comment is not updated.

[Copilot]

Pull Request Overview

This PR improves the accuracy of the lines of code (LOC) comparison in the CI workflow by changing the comparison baseline from the PR base branch to the merge base commit between the PR and base branches. This prevents inaccurate results when the PR branch is not up-to-date with the main branch.

• Updates the checkout process to first checkout the PR code, then find and checkout the merge base commit
• Adds a new step to calculate the merge base between head and base branches using git merge-base
• Replaces direct base branch comparison with merge base comparison for more accurate LOC reporting

[Copilot]

The git merge-base command could fail if the branches don't share common history or if the fetch operations didn't retrieve sufficient commits. Add error handling to check if the merge base was found successfully.

Suggested change

	MERGE_BASE=$(git merge-base origin/$HEAD_REF origin/$BASE_REF)
	MERGE_BASE=$(git merge-base origin/$HEAD_REF origin/$BASE_REF || echo "")
	if [ -z "$MERGE_BASE" ]; then
	echo "Error: Unable to find a merge base between $HEAD_REF and $BASE_REF." >&2
	exit 1
	fi

[github-actions]

Lines of code report

Total lines added: 1
Total lines removed: 0
Total lines changed: 1

Detailed view

+-------------------------------------------------+-------+------+
| File                                            | Lines | Diff |
+-------------------------------------------------+-------+------+
| ethrex/crates/l2/sequencer/l1_proof_verifier.rs | 270   | +1   |
+-------------------------------------------------+-------+------+