Fix: stale forks fail the signature check and get told to remove other people's files

[geetxnshgoyal]

Bug

The check compared current main with the PR head (git diff base head). A student who forked before other signatures were merged, like Prince in #44, saw every newer signature listed as a deletion in their PR. They failed with "This PR changes 8 files", and the bot told them to remove other people's files.

Fix

One line: git diff base...head (three dots) compares against the merge base, the point where the student's branch started. That's exactly what the PR's Files changed tab shows.

Tested

• New case: a student branches, main gains two other signatures, then the PR is checked. It now shows only A signatures/<user>.md and passes. The old code listed the other signatures as deletions, which matches the Sign the workbook: Prince sharma #44 screenshot.
• All 9 existing cases still pass.

This PR's own check fails by design (it isn't a signature), so it needs an admin merge. After merging, re-run the failed checks on open PRs so the bot corrects any wrong comments.

[github-actions]

Hi @geetxnshgoyal 👋 thanks for signing the workbook! The check found a problem:

• Your file must be named exactly signatures/geetxnshgoyal.md — your GitHub username, capitals included. Yours is .github/workflows/validate-signature.yml. Fix it with: git mv ".github/workflows/validate-signature.yml" signatures/geetxnshgoyal.md

How to fix it (in your clone, on the same branch as this PR):

# fix the file as described above, then:
git add -A
git commit -m "Fix signature"
git push

Your file should look like:

# Your Real Name

- **GitHub:** @geetxnshgoyal
- **Batch:** 2026
- **I'm here to:** your own line
- **One thing I've built:** a link, or nothing yet

Push to this same PR. Don't open a new one. This comment updates itself each time the check re-runs.