Allow configuring different face for diff-hl-flydiff-mode

[pksadiq]

I didn't see a key to customize face for flydiff changes, ie, the changes in file that are not saved yet. It shall be nice to support that.

Thanks

[dgutov]

diff-hl-flydiff-mode doesn't know which changes are saved, and which aren't.

Adding a feature like that would require more than a few new faces.