better way for open MR/PR

[hotoo]

When I open new MR/PR, gitopen is use current working branch and remote default branch for now.
but some times, default branch isn't I want, and I can't change the default branch,

Way I

1. auto get the base branch which the current working branck.
2. gitopen mr base-branch:current-working-branch

Way II

1. Get the remote branchs: git branch -r
2. If just one remote branch, use it as base branch
3. Else, show remote branches option menu.

[hotoo]

• http://zsh.sourceforge.net/Guide/zshguide06.html
• https://github.com/zsh-users/zsh-completions
• http://blog.bhe.me/blog/2014/03/17/first-step-on-writing-zsh-completion-functions/
• http://tommy.net.cn/2015/02/24/enable-zsh-completion-of-brew-under-mac-os-x/