RFR: 2197: Make the /backport command support a "repo:branch" argument

[Zhao Song]

This patch is trying to make /backport command support `repo:branch` argument.

And as ErikD suggested, `repo` should be optional if the user is backporting to a different branch in the same repository.

> To make it slightly more convenient to backport a commit to a branch in the same repository as the commit is residing in we would allow short form "/backport :branch" (this is also fully backwards compatible and cannot become ambiguous in the future).

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Commit messages:
 - SKARA-2197

Changes: https://git.openjdk.org/skara/pull/1620/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1620&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2197
  Stats: 63 lines in 3 files changed: 56 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/skara/pull/1620.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1620/head:pull/1620

PR: https://git.openjdk.org/skara/pull/1620