RFR: 344: PR should warn when source branch is an upstream branch

[Erik Helin]

Hi all,

please review this patch that warns contributors that creates a poll request
from a branch that is already present in the upstream repository. This is rarely
what you want to do and therefore we should issue a warning.

Testing:
- `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - skara-344

Changes: https://git.openjdk.java.net/skara/pull/555/files
 Webrev: https://webrevs.openjdk.java.net/skara/555/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-344
  Stats: 316 lines in 10 files changed: 309 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/555.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/555/head:pull/555

PR: https://git.openjdk.java.net/skara/pull/555