Generate a file called `.keep` instead of `.gitkeep`.

[sanmai-NL]

diesel migration generate

creates a .gitkeep as well. Do not do this if the crate isn’t under Git VCS.

[sgrif]

I guess we can call the file .keep.

[ocamlmycaml]

I’d like to take a stab at this issue. This would be my first rust project contribution here so i’m hoping this will be ok for that as well

[joelgallant]

If this was changed, could it also remove the old .gitkeep file?

[ocamlmycaml]

If this was changed, could it also remove the old .gitkeep file?

hmm is that a good idea? it'll be dead code after you use diesel_cli once (in the newer release). I guess it's ok if I can add a TODO or issue to clean it up in a much later release