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.
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Co-authored-by: Jonathan Tran <jonnytran@gmail.com>
2b3f12f6fd