RFR: vcs: support explicit fast-forward merging

[Erik Helin]

Hi all,

please review this patch that makes it explicit for `Repository.merge` whether a fast-forward merge should be attempted
or not.

Testing:
- [x] `make test` passes on Linux x64
- [x] Added two new unit tests

Thanks,
Erik

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

Commit messages:
 - hg: support non-fast-forward merging

Changes: https://git.openjdk.java.net/skara/pull/731/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=731&range=00
  Stats: 122 lines in 5 files changed: 100 ins; 0 del; 22 mod
  Patch: https://git.openjdk.java.net/skara/pull/731.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/731/head:pull/731

PR: https://git.openjdk.java.net/skara/pull/731