Why when I checkout a new branch using Git and push this new branch to origin repo, the base commit ID is 0000000000000000000000000000000000000000?

I’m using update hook in Git to check if this push has been approved in Phabricator(By connecting to mysql). When I trying to use the second param as old commitID in update script, I always get 40 0s like 0000000000000000000000000000000000000000 when this reference(branch) was created by git checkout -b master and not pushed before.
But I want to get the last commitID of master to demonstrate the starting point of this reference.

like this:

last commitID of my master branch is 978881526a1be5dc49aeef93cd83679c2142eb60
I’m using master branch to checkout a new branch like this git checkout -b t1
and then I code something and commit some times, so I got a new head of branch:dfb3094b7e9f06a8bf49671b5bbab45dd5c4318e
finally, I push this branch to origin using git push origin t1

I expected to get the params of update script as below:

$1 refs/xxx/t1

$2 978881526a1be5dc49aeef93cd83679c2142eb60

$3 dfb3094b7e9f06a8bf49671b5bbab45dd5c4318e

but I got them as below:

$1 refs/xxx/t1

$2 0000000000000000000000000000000000000000

$3 dfb3094b7e9f06a8bf49671b5bbab45dd5c4318e

Does anyone know how to solve this problem?

Git Baby is a git and github fan, let's start git clone.