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/hooks/post-receive.d/gitea

@@ -1,2 +1,2 @@
 #!/usr/bin/env bash
-"/home/tris/Projects/go/src/code.gitea.io/gitea/gitea" hook --config='/home/tris/Projects/go/src/code.gitea.io/gitea/custom/conf/app.ini' post-receive
+"$GITEA_ROOT/gitea" hook --config="$GITEA_ROOT/$GITEA_CONF" post-receive

tests/gitea-repositories-meta/user2/repo20.git/hooks/pre-receive.d/gitea

@@ -1,2 +1,2 @@
 #!/usr/bin/env bash
-"/home/tris/Projects/go/src/code.gitea.io/gitea/gitea" hook --config='/home/tris/Projects/go/src/code.gitea.io/gitea/custom/conf/app.ini' pre-receive
+"$GITEA_ROOT/gitea" hook --config="$GITEA_ROOT/$GITEA_CONF" pre-receive

tests/gitea-repositories-meta/user2/repo20.git/hooks/update.d/gitea

@@ -1,2 +1,2 @@
 #!/usr/bin/env bash
-"/home/tris/Projects/go/src/code.gitea.io/gitea/gitea" hook --config='/home/tris/Projects/go/src/code.gitea.io/gitea/custom/conf/app.ini' update $1 $2 $3
+"$GITEA_ROOT/gitea" hook --config="$GITEA_ROOT/$GITEA_CONF" update $1 $2 $3

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

@@ -0,0 +1 @@
+x
•ŽAnÃ0sÖ+ø<>”l‘	Ð[_A‹TkIJC>ø÷Õz[,f1›·Zç!øKÛÍ€“èSð<53>L5[,©DÒ‰'<27>:aˆRнe·µÁDlã È:^C<0F>g”lHƒd<C692>æ‡Â>iqr´ßm‡ïs1ø‚ÛK<C39B>º­m=?Uæå3oõž˜®<CB9C>òð<C3B2>Ñõ¶¿köß<C3B6>{ª‚@wÔʼ˜û<03>E]

tests/gitea-repositories-meta/user2/repo20.git/objects/c8/e31bc7688741a5287fcde4fbb8fc129ca07027

@@ -0,0 +1,2 @@
+x
•ÎA
+Â0@Q×9Å\@™NÒ&
wž"“L´Ø4Ò¦‚·×+¸ýðàÇZÊØ€ˆvm<11>`ÉÙ!&ÇuÖŽmò¾÷FKÇl³·aÈê™8t¨]¢l;ÆÀHè}g´<67>9'2}´{<7B>*líQ¸}&<26>+Ÿi+unóv¾—0N‡XË	ºÁŽ,!Â{Dõ«¿»&ÿ:uI	š¬
âú†<N¢¾¨qEo

tests/gitea-repositories-meta/user2/repo20.git/objects/cf/e3b3c1fd36fba04f9183287b106497e1afe986

@@ -0,0 +1,3 @@
+x
•ŽAnÂ0EYűs
<01>=v2FBv=D5¶';U<>Tâöř
+Ýţ§÷ôóZ묀ş‰€%›P(z“—
ŁźŠpńDě%8¶!8[Ě/oŇrďR¦1FępŔHS.¦”┞3÷$’á]ëßďEŕ—gŮëÚ´í×{ĺy9ĺµ~<7E>{	Ď
Žv°ÖôµżSůŻgn˛<6E>
+¨Ľô”_Ŕ­Ŕ2·çĎc6tuI‚

tests/gitea-repositories-meta/user2/repo20.git/refs/heads/add-csv

@@ -0,0 +1 @@
+c8e31bc7688741a5287fcde4fbb8fc129ca07027

tests/gitea-repositories-meta/user2/repo20.git/refs/heads/remove-files-a

@@ -0,0 +1 @@
+cfe3b3c1fd36fba04f9183287b106497e1afe986

tests/gitea-repositories-meta/user2/repo20.git/refs/heads/remove-files-b

@@ -0,0 +1 @@
+8babce967f21b9dfa6987f943b91093dac58a4f0