Change detection using git "three dot" diff (#35)

* Rework change detection via `git diff`

Previous implementation performed simple diff between two versions. New implementation fetches on demand more commits to have the merge base between two branches. Now it will detect only changes introduced by branch that was pushed, instead of mixing with changes introduced meanwhile on the base branch.

.github/workflows/pull-request-verification.yml

@@ -75,7 +75,12 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
-    - run: touch add.txt && rm README.md && echo "TEST" > LICENSE && git add -A
+    - name: configure GIT user
+      run: git config user.email "john@nowhere.local" && git config user.name "John Doe"
+    - name: modify working tree
+      run: touch add.txt && rm README.md && echo "TEST" > LICENSE
+    - name: commit changes
+      run: git add -A && git commit -a -m 'testing this action'
     - uses: ./
       id: filter
       with: