RFR: merge-bot: update message for failed merges

[Erik Helin]

Hi all,

please review this patch that updates the merge bot to produce a nicer message
(and clearer instructions) in the failed merge pull requests. The message will
now include the files that contains merge conflicts. I also updated the message
to limit the number of files and commits it lists (currently capped to ten
each). Finally I also fixed the instructions for fixing a merge conflict to be
correct ��

Testing:
- added unit test for `ReadOnlyRepository::status`
- `make test` passes on Linux x64

Thanks,
Erik

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

Commits:
 - 176d03a4: merge-bot: update message for failed merges

Changes: https://git.openjdk.java.net/skara/pull/416/files
 Webrev: https://webrevs.openjdk.java.net/skara/416/webrev.00
  Stats: 134 lines in 8 files changed: 120 ins; 4 del; 10 mod
  Patch: https://git.openjdk.java.net/skara/pull/416.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/416/head:pull/416

PR: https://git.openjdk.java.net/skara/pull/416