Simplify `gpull` and `gpush` functions

[ezhlobo]

We can simplify these functions:

https://github.com/datarockets/dotfiles/blob/8122aeb65d4b4cea78beb0300817dc7899e65aec/zsh/functions/git.zsh#L1-L5

https://github.com/datarockets/dotfiles/blob/8122aeb65d4b4cea78beb0300817dc7899e65aec/zsh/functions/git.zsh#L7-L11

By replacing $current_branch with HEAD.

Recently I've been told by @atatarenko that there is an alias for current branch — HEAD.

Proof on https://git-scm.com

With this we can simplify it.

Other resources to consider before making such decisions:

• https://stackoverflow.com/questions/12532913/git-pull-origin-head
• https://coderwall.com/p/xuq5cq/git-push-current-branch