Set filetype for git_blame window

[seblyng]

I had an idea to fix gitmoji for the git blame window, but my solution depends on setting a few settings in the ftplugin directory. A filetype for the window would help with this. I could fix a PR for this as this would only require one line.

If there is a way for me to set the filetype myself, this would also work! I just couldn't figure out a way to do this

[lewis6991]

There have been one or two requests for configuring preview windows so something more general would be preferable. Maybe something like an on_open_win callback.

[seblyng]

Yeah I think that's a good idea!

[lewis6991]

This should be fulfilled by #622 which sets the window variable gitsigns_preview to either blame or hunk.