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
<01><>An<41>0s<>+<2B><><16>l<EFBFBD>	<09>[_A<5F>TkIJC><3E><><EFBFBD>z[,f1<66><31>Z<EFBFBD>!<21>K<EFBFBD>̀<EFBFBD><CD80>S<><53>L5[,<14>D҉'<27>:a<>Rнe<D0BD><65><EFBFBD>Dl<44> <20>:^C<0F>g<EFBFBD>lH<6C>d<EFBFBD><64><EFBFBD><EFBFBD>>iqr<71><72>m<EFBFBD><6D>s1<73><31><EFBFBD>K<EFBFBD><4B><EFBFBD>m=?U<><55>3o<33><0E><><EFBFBD><EFBFBD><07><><EFBFBD><11><><EFBFBD><EFBFBD>k<EFBFBD>ߝ{<7B><>@w<>ʼ<><CABC><03>E]

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><>

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>