RFR: mirror-bot: use to repo to restrict concurrency

[Erik Helin]

Hi all,

please review this patch that makes the mirror bot use the `to` repository for
determining if whether multiple mirror work items can run concurrently. This
means the also must the `to` directory for creating its storage.

Testing:
- `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - mirror-bot: use to repo to restrict concurrency

Changes: https://git.openjdk.java.net/skara/pull/594/files
 Webrev: https://webrevs.openjdk.java.net/skara/594/webrev.00
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/594.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/594/head:pull/594

PR: https://git.openjdk.java.net/skara/pull/594