Make J omit comment leader of joined line

[ghost]

Make J combine lines like

// line 1
// line 2

to

// line 1 line 2

instead of

// line 1 // line 2

[JesseKPhillips]

Sorry for not responding quickly.

I have reservations on this change because it isn't a syntax rule. I think it would make more sense in the indent file.

[ghost]

It's not really about indentation, it's just about people never wanting those extra slashes when they join two comment lines. I don't think there is a more logical place for it (rust.vim also has this in in ftplugin/ btw).