RFR: merge-bot: allow specs to define dependencies

[Erik Helin]

Hi all,

please review this patch that allow the specifications for the merge bot to:

1. Have a name
2. Specify the names of other specifications as dependencies

The above means that the merge bot now can express a bit more complicated
merges, such as "only merge the branch `feature` into branch `demo` if the merge
of branch `feature-vm` into branch `feature` _and_ the merge of branch
`feature-lang` into branch `feature` succeeded".

Testing:
- `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - merge-bot: allow specs to define dependencies

Changes: https://git.openjdk.java.net/skara/pull/600/files
 Webrev: https://webrevs.openjdk.java.net/skara/600/webrev.00
  Stats: 59 lines in 2 files changed: 52 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/600.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/600/head:pull/600

PR: https://git.openjdk.java.net/skara/pull/600