Allow detect whether it's in a database transaction for a context.Context (#21756)

Fix #19513

This PR introduce a new db method `InTransaction(context.Context)`,
and also builtin check on `db.TxContext` and `db.WithTx`.
There is also a new method `db.AutoTx` has been introduced but could be used by other PRs.

`WithTx` will always open a new transaction, if a transaction exist in context, return an error.
`AutoTx` will try to open a new transaction if no transaction exist in context.
That means it will always enter a transaction if there is no error.

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: 6543 <6543@obermui.de>

models/git/branches_test.go

@@ -102,7 +102,7 @@ func TestRenameBranch(t *testing.T) {
 	repo1 := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 1})
 	_isDefault := false
 
-	ctx, committer, err := db.TxContext()
+	ctx, committer, err := db.TxContext(db.DefaultContext)
 	defer committer.Close()
 	assert.NoError(t, err)
 	assert.NoError(t, git_model.UpdateProtectBranch(ctx, repo1, &git_model.ProtectedBranch{