RFR: 1111: Backport command need better matching of target repo

[Erik Joelsson]

This patch improves the /backport commit command, making it smarter when trying to find the target repository from the argument given by the user. Instead of trying to piece it together based on the namespace (organization/group) of the current repository, it goes straight to the forks configuration and tries to match the repo from there. We can only do backports if there is a valid fork configuration anyway.

In addition to this, I'm also changing the error message you get when you enter an invalid target repo. The list now includes the namespace prefix for each repo, and a note that supplying it is optional.

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Commit messages:
 - SKARA-1111

Changes: https://git.openjdk.java.net/skara/pull/1204/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1204&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1111
  Stats: 48 lines in 4 files changed: 10 ins; 17 del; 21 mod
  Patch: https://git.openjdk.java.net/skara/pull/1204.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1204/head:pull/1204

PR: https://git.openjdk.java.net/skara/pull/1204