RFR: merge-bot: clearer merge messages when merging between repos

[Erik Helin]

Hi all,

please review this small patch that makes the commit message a bit clearer when
the merge bot merges branches in _different_ repositories. For example, if the
bot merges openjdk/jdk:master to openjdk/panama-foreign:master, the message will
now be:

> Automatic merge of jdk:master into master

Testing:
- `make test` passes on Linux x64
- Updated unit tests to test for new message style

Thanks,
Erik

-------------

Commits:
 - e6c53da6: merge-bot: clearer merge messages when merging between repos

Changes: https://git.openjdk.java.net/skara/pull/407/files
 Webrev: https://webrevs.openjdk.java.net/skara/407/webrev.00
  Stats: 10 lines in 2 files changed: 3 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/407.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/407/head:pull/407

PR: https://git.openjdk.java.net/skara/pull/407