RFR: vcs: support explicit fast-forward merging

[Robin Westberg]

On Sat, 22 Aug 2020 09:25:16 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> 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

Looks good!

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Marked as reviewed by rwestberg (Reviewer).

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