RFR: merge-bot: give each branch a new number as name

[Erik Helin]

Hi all,

please review this patch that makes the merge bot always increment its branch
names by one. The previous code (mistakenly) used the number of open pull
request, but that is not monotonically increasing. Since the merge bot does not
delete branches it has created, the number of branches should be monotonically
increasing ��

Testing:
- `make test` on Linux x64 passes

Thanks,
Erik

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Commit messages:
 - merge-bot: give each branch a new number as name

Changes: https://git.openjdk.java.net/skara/pull/579/files
 Webrev: https://webrevs.openjdk.java.net/skara/579/webrev.00
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/579.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/579/head:pull/579

PR: https://git.openjdk.java.net/skara/pull/579