More robust `crux-rename-file-and-buffer`.

[wyuenho]

Currently if you rename a new buffer that hasn't been saved yet, crux-rename-file-and-buffer will only rename the buffer but not reset the visited file name, so saving after renaming will still save to the file name before renaming. The PR also let the user know if we are renaming to an existing buffer.

• Offer to save file first if modified or new unsaved buffer.
• Message user if new file name is the same.

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Thanks!

[wyuenho]

@bbatsov how does this look?

[bbatsov]

The messages can be a bit clearer, but your proposal looks good to me overall.

[wyuenho]

Do you have a better message? What's unclear?

[wyuenho]

@bbatsov ping.