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/cf/e3b3c1fd36fba04f9183287b106497e1afe986

@@ -0,0 +1,3 @@
+x
<01><>An<41>0EY<45>s
<01>=v2FBv=D5<44>';U<>T<EFBFBD><54><EFBFBD>
+<EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>Z묀<0F><><EFBFBD>%<25>P(z<1E><>
<1D><><EFBFBD>p<EFBFBD>D<EFBFBD>%8<>!8[<5B>/o<>r<14>R<EFBFBD>1F<31>p<EFBFBD>HS.<12><><EFBFBD><EFBFBD><1D>3<EFBFBD>$<24><>]<1F><06><>E<EFBFBD><06>g<EFBFBD><67>ڴ<EFBFBD><DAB4>{<7B>y9<79><39>~<7E>{	<09>
<01>v<EFBFBD><76><EFBFBD><EFBFBD><EFBFBD>S<EFBFBD><53>gn<67><6E>
+<EFBFBD><EFBFBD><EFBFBD><EFBFBD>_<EFBFBD><EFBFBD><EFBFBD>2<EFBFBD><EFBFBD><EFBFBD>c6tuI<75>