:Commits command: allow users to change sink?

As far as I understand, now you can't redefine actions for `:Commits`/`:BCommits` without changing `fzf.vim/autoload/fzf/vim.vim`, am I correct?

If so -- and since `:Commits` relies on the external dependency, vim-fugitive, which may or may not be installed -- it would be great to allow users to set the sink actions via some configurable variable may be? And if no such variable is defined, then use the default predefined sink, `s:commits_sink()`.

What do you think?

Read fzf.vim/autoload/fzf/vim.vim and trace the :Commits and :BCommits definitions, including the default s:commits_sink() and their vim-fugitive dependency. Done means users can configure the sink actions without editing that file, while the existing sink remains the fallback when no variable is defined.

Written by the indexing model from the issue text.