RFR: vcs: do not hard-code VCS default branch names [v2]

[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

Erik Helin has updated the pull request incrementally with one additional commit since the last revision:

  Use abstraction for HgRepository as well

-------------

Changes:
  - all: https://git.openjdk.java.net/skara/pull/806/files
  - new: https://git.openjdk.java.net/skara/pull/806/files/6b2063aa..85f2b95f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=806&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=806&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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