Git flow finish feature with rebase option errors with 'flow is not a git command'

When finishing a feature and selecting the 'Rebase' option, an error is reported:

git: 'flow' is not a git command

This happens regardless of whether the embedded or system git options are used. Unchecking the 'Rebase' option completes correctly.

Originally raised here: https://answers.atlassian.com/questions/83511/sourcetree-cannot-find-git-flow-osx-mountain-lion