Use beforeCommit instead of baseCommit (#22949)

Replaces: https://github.com/go-gitea/gitea/pull/22947
Fixes https://github.com/go-gitea/gitea/issues/22946
Probably related to https://github.com/go-gitea/gitea/issues/19530

Basically, many of the diffs were broken because they were comparing to
the base commit, where a 3-dot diff should be comparing to the [last
common
ancestor](https://matthew-brett.github.io/pydagogue/git_diff_dots.html).

This should have an integration test so that we don’t run into this
issue again.

---------

Co-authored-by: Jonathan Tran <jonnytran@gmail.com>

models/db/iterate_test.go

@@ -25,7 +25,7 @@ func TestIterate(t *testing.T) {
 		return nil
 	})
 	assert.NoError(t, err)
-	assert.EqualValues(t, 81, repoCnt)
+	assert.EqualValues(t, 83, repoCnt)
 
 	err = db.Iterate(db.DefaultContext, nil, func(ctx context.Context, repoUnit *repo_model.RepoUnit) error {
 		reopUnit2 := repo_model.RepoUnit{ID: repoUnit.ID}

models/fixtures/repo_unit.yml

@@ -556,3 +556,16 @@
   repo_id: 54
   type: 1
   created_unix: 946684810
+
+-
+  id: 82
+  repo_id: 31
+  type: 1
+  created_unix: 946684810
+
+-
+  id: 83
+  repo_id: 31
+  type: 3
+  config: "{\"IgnoreWhitespaceConflicts\":false,\"AllowMerge\":true,\"AllowRebase\":true,\"AllowRebaseMerge\":true,\"AllowSquash\":true}"
+  created_unix: 946684810