RFR: 1466: Empty backport causes endless retries of command

[Erik Joelsson]

If a user tries to /backport a commit that results in an empty change, the BackportCommand fails to realize this and just errors out. It will then endlessly retry the command. This patch makes the BackportCommand detect an empty change and report it to the user.

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Commit messages:
 - SKARA-1466

Changes: https://git.openjdk.java.net/skara/pull/1329/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1329&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1466
  Stats: 50 lines in 2 files changed: 50 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/1329.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1329/head:pull/1329

PR: https://git.openjdk.java.net/skara/pull/1329