GitSquashMergeBuilder

Sets the fast-forward mode to use when executing git merge. The modes available are documented on the enumeration and have different behaviour when applied to --squash merges:

• --no-ff and --ff are both ignored
  • --ff is accepted by git merge --squash but does not change its behaviour
  • --no-ff causes git merge --squash to fail but mimics the behaviour of a squash merge, which always produces a new commit, and so it is ignored by this builder
• --ff-only when paired with --squash still creates a new commit, but will fail if the source branch is not fast-forward from the target rather than actually performing any merging