Make Gitea always use its internal config, ignore /etc/gitconfig (#33076)

In history, Gitea could use the system config `/etc/gitconfig` because
some users said that "they might put certNonceSeed in it"

Actually, we shouldn't not use it, because it also causes conflicts
(there are already some fixes like #28848)

To make the system clear, I think it's worth to introduce the breaking
change: add `GIT_CONFIG_NOSYSTEM=1` to all git commands.


## ⚠️ BREAKING ⚠️

 For most users, nothing need to do.

If you have made changes to `/etc/gitconfig` to affect Gitea's behavior,
you need to move these config options to Gitea's internal git config
file, it is usually in Gitea's `{AppDataPath}/home/.git` directory.

models/unittest/testdb.go

@@ -59,13 +59,7 @@ func InitSettings() {
 	_ = hash.Register("dummy", hash.NewDummyHasher)
 
 	setting.PasswordHashAlgo, _ = hash.SetDefaultPasswordHashAlgorithm("dummy")
-	setting.InitGiteaEnvVars()
-
-	// Avoid loading the git's system config.
-	// On macOS, system config sets the osxkeychain credential helper, which will cause tests to freeze with a dialog.
-	// But we do not set it in production at the moment, because it might be a "breaking" change,
-	// more details are in "modules/git.commonBaseEnvs".
-	_ = os.Setenv("GIT_CONFIG_NOSYSTEM", "true")
+	setting.InitGiteaEnvVarsForTesting()
 }
 
 // TestOptions represents test options