Improve fixup commits script

This script is failing currently on
https://github.com/jesseduffield/lazygit/pull/3631 because that fork's
master branch is 300 commits behind our own, but the feature branch is
up to date.

The thing is, we don't actually need to involve the master branch. All
we care about is the feature branch's own commits, so this commit simply
fetches those commits and checks them.

.github/workflows/ci.yml

@@ -225,8 +225,8 @@ jobs:
     if: github.ref != 'refs/heads/master'
     steps:
       # See https://github.com/actions/checkout/issues/552#issuecomment-1167086216
-      - name: "PR commits + 1"
-        run: echo "PR_FETCH_DEPTH=$(( ${{ github.event.pull_request.commits }} + 1 ))" >> "${GITHUB_ENV}"
+      - name: "PR commits"
+        run: echo "PR_FETCH_DEPTH=$(( ${{ github.event.pull_request.commits }} ))" >> "${GITHUB_ENV}"
 
       - name: "Checkout PR branch and all PR commits"
         uses: actions/checkout@v4
@@ -235,10 +235,6 @@ jobs:
           ref: ${{ github.event.pull_request.head.ref }}
           fetch-depth: ${{ env.PR_FETCH_DEPTH }}
 
-      - name: "Fetch the other branch with enough history for a common merge-base commit"
-        run: |
-          git fetch origin ${{ github.event.pull_request.base.ref }}
-
       - name: Check for fixups
         run: |
           ./scripts/check_for_fixups.sh ${{ github.event.pull_request.base.ref }}