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>

tests/gitea-repositories-meta/user2/repo20.git/objects/8b/abce967f21b9dfa6987f943b91093dac58a4f0

@@ -0,0 +1 @@
+x
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