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/c8/e31bc7688741a5287fcde4fbb8fc129ca07027

@@ -0,0 +1,2 @@
+x
<01><>A
+<EFBFBD>0@Q<>9<EFBFBD>\@<40>N<EFBFBD>&
w<>"<22>L<EFBFBD><4C>4Ҧ<34><D2A6><EFBFBD>+<2B><><EFBFBD><EFBFBD><EFBFBD>Z<EFBFBD>؀<EFBFBD>vm<11>`<08><>!&<26>u֎m<D68E><6D><EFBFBD>FK<>l<EFBFBD><6C>a<EFBFBD><61><16>8t<38>]<5D>l;<3B><>H<EFBFBD>}g<><67>9'2}<7D>{<7B>*l<>Q<17>}&<26>+<1C>i+un<75>v<EFBFBD><76>0N<30>X<EFBFBD>	<09><><0E>,!<21>{D<><44><EFBFBD><EFBFBD>&<26>:uI	<09><>