RFR: forge: support project id for GitLab repos

[Erik Helin]

Hi all,

please review this patch that makes `GitLabRepository` work with project id:s as
well as project names. I had to make a few changes to the `GitLabRepository`
constructor and add a new `getProjectInfo` overload to `GitLabHost`.

Testing:
- `make test` on Linux x64

Thanks,
Erik

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Commit messages:
 - forge: support project id for GitLab repos

Changes: https://git.openjdk.java.net/skara/pull/574/files
 Webrev: https://webrevs.openjdk.java.net/skara/574/webrev.00
  Stats: 17 lines in 3 files changed: 14 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/574.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/574/head:pull/574

PR: https://git.openjdk.java.net/skara/pull/574