add git_current_branch function

[not-reed]

I recently switched from oh-my-zsh to this, but ran into an issue running gpsup

zsh: command not found: git_current_branch
fatal: The current branch dev has no upstream branch.
To push the current branch and set the remote as upstream, use

    git push --set-upstream origin dev

I pulled this two functions over from lib/git.zsh and its not working as expected for me. As far as I could tell, the current_branch function isn't used anywhere so I removed it also

[teschmitt]

Wow you've fixed literally the only problem I've had with this plugin and this PR has been idling here for almost 2 years. Thank you so much for this fix, I was about to dive into the original plugin internals to do it myself.