RFR: mirror-bot: support mirroring of selected branches

[Erik Helin]

Hi all,

please review this patch that enables the mirror bot to mirror only selected
branches (instead of all as is done today). This is useful for projects who
might have a `master` that is a replica of `jdk:master`, but don't want tags
(nor potentially other branches) from the
[jdk](https://git.openjdk.java.net/jdk) repository.

Testing:
- `make test` passes on Linux x64
- Added a new unit test

Thanks,
Erik

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Commit messages:
 - mirror-bot: support mirroring of selected branches

Changes: https://git.openjdk.java.net/skara/pull/536/files
 Webrev: https://webrevs.openjdk.java.net/skara/536/webrev.00
  Stats: 92 lines in 3 files changed: 85 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/536.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/536/head:pull/536

PR: https://git.openjdk.java.net/skara/pull/536