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

[Magnus Ihse Bursie]

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

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

Changes: https://git.openjdk.java.net/skara/pull/1083/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1083&range=00
  Stats: 13 lines in 2 files changed: 6 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/1083.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1083/head:pull/1083

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