Enable fetching git default branch dynamically instead of settings

[keymastervn]

Given the context that I have been working on multiple repos with different default branches, I've recently faced errors no on uncommitted branches.

The cmd of Github: Blame (default branch) calling to main concretely instead of master or development (depending on which repo) is untrue.

By configuring "default_branch": "", this PR is now dynamically allowing us to fetch the true default branch which is defined by the active git repo. Hence we can enjoy Github: Blame (default branch) when working in pre-push style.