Preview `jbuilder promote` changes

[samoht]

Currently the only way (that I found) to preview the changes done by promote is to do something like: jbuilder promote && git diff which is not really optimal (especially if I am not really sure if I want to promote these changes).

Instead, it would be great to have something like jbuilder promote --preview which will run the diff program between the .corrected files.

[rgrinberg]

Sounds like a good idea. However, this should really be its own command as its not actually promoting anything.

[emillon]

Duplicate of #5368

[rgrinberg]

Let's continue the discussion in #5368 as it's a little fuller.