RFR: 428: git-jcheck does not work on repositories without default or maser branch

[Erik Helin]

Hi all,

please review this small patch that makes `JCheck` work on repositories _without_ a remotee `HEAD` ref being present
(i.e. `refs/remotes/origin/HEAD`) and _without_ a `master` ref being present (i.e. `refs/heads/master`). In such a case
`JCheck` will now fall back to configuration values for allowed tags and branches as specified in the configuration
file at `HEAD` (`HEAD` is always present).

Testing:
- [x] `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - skara-428

Changes: https://git.openjdk.java.net/skara/pull/676/files
 Webrev: https://webrevs.openjdk.java.net/skara/676/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-428
  Stats: 5 lines in 1 file changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/676.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/676/head:pull/676

PR: https://git.openjdk.java.net/skara/pull/676