Mark PR reviews as stale at push and allow to dismiss stale approvals (#9532)

Fix #5997.

If a push causes the patch/diff of a PR towards target branch to change, all existing reviews for the PR will be set and shown as stale.
New branch protection option to dismiss stale approvals are added.
To show that a review is not based on the latest PR changes, an hourglass is shown

modules/git/repo_compare.go

@@ -112,3 +112,9 @@ func (repo *Repository) GetPatch(base, head string, w io.Writer) error {
 	return NewCommand("format-patch", "--binary", "--stdout", base+"..."+head).
 		RunInDirPipeline(repo.Path, w, nil)
 }
+
+// GetDiffFromMergeBase generates and return patch data from merge base to head
+func (repo *Repository) GetDiffFromMergeBase(base, head string, w io.Writer) error {
+	return NewCommand("diff", "-p", "--binary", base+"..."+head).
+		RunInDirPipeline(repo.Path, w, nil)
+}