aliases/git: Move git plugin aliases to their own file

[RobLoach]

In an attempt to fix the issue brought up in #470, this moves the aliases defined in the git plugin to their own git aliases file.

It allows having the git shell functions, while keeping the git aliases disabled.

[akinomyoga]

Hmm, I'm opposed to this change. A significant number of users use this plugin since it is enabled by default in Oh My Bash (OMB). So the compatibility issue that I raised in #602 applies here even more . Even if we forget about the compatibility issue, this breaks correspondence to the upstream OMZ plugin git.

Also, many of the left functions are actually helper functions used to implement the aliases. These aliases depend on shell functions, such as git_{current,main,develop}_branch. I also suspect that the other functions are for similar purposes. In that sense, this move of the partial content breaks the aliases relying on them.

[RobLoach]

Sounds good with me 👍