PSA: use file-by-file config for external difftools to handle rename

[evmar]

I have been happily using difft for a while, but only today finally tracked down that the proper way to configure it for it to display renames properly is these settings:

[ui]
diff.tool = "difft"

[merge-tools.difft]
diff-args = ["--color=always", "$left", "$right"]
diff-invocation-mode = "file-by-file"

[evmar]

BTW, I had this wrong because difft docs say otherwise:

https://difftastic.wilfred.me.uk/jj.html

The problem is that unless you specify file-by-file, it doesn't know about file renames.
But if you do specify file-by-file, the paths (on Mac) are long ugly /var/tmp paths.
I wonder if there's some better way of doing it that is the best of all worlds.

[CobaltCause]

Related, for difftastic in particular: Wilfred/difftastic#863

[evmar]

You inspired me to send a PR, let's figure this out!

Wilfred/difftastic#864