RFR: vcs: do not hard-code VCS default branch names

[Erik Helin]

Hi all,

please review this patch that ensures that we don't hard-code the name of the default branches for various version
control system throughout the Skara source code. Skara has from the beginning been very strict about abstracting away
VCS implementation details, but the name of the default branches has managed to find its way into multiple corners of
the source code.

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

Thanks,
Erik

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Commit messages:
 - vcs: do not hardcode VCS default branch names

Changes: https://git.openjdk.java.net/skara/pull/806/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=806&range=00
  Stats: 56 lines in 19 files changed: 22 ins; 1 del; 33 mod
  Patch: https://git.openjdk.java.net/skara/pull/806.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/806/head:pull/806

PR: https://git.openjdk.java.net/skara/pull/806