Allow configuring the keybinding prefix

[kyanagi]

fzf-git uses C-g as the keybinding prefix, but I would like to keep C-g for its original use.
How about making the keybinding prefix configurable?

For zsh, I have implemented a way to set the prefix using an environment variable FZF_GIT_KEY_PREFIX, as shown below.
kyanagi@8ef4a8c
I intended to implement this for bash as well, but I couldn't get it to work, so the implementation is currently limited to zsh only.