RFR: Add -t/--tool to pr show to use git difftool

[Magnus Ihse Bursie]

On Fri, 19 Mar 2021 12:49:01 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> This will leverage the already existing `git pr show` to use your favorite diff viewer.

There are no tests for the CLI; I have done adhoc testing on my computer to verify that it works.

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PR: https://git.openjdk.java.net/skara/pull/1083