Previously: replace the YNode (yaml.Node) with a new one which is just
the replacement string.
Now: change the value of the YNode, and set it back in place.
When I set my default branch for `git init` in ~/.gitconfig, tests
start failing. This is because go-git assumes a default of `master`
when e.g., pulling commits, but exec'ing git (as the test server does,
via gitkit) will get you a different branch.
The answer is to be explicit about the branch created in tests, and in
all operations like cloning. The natural end point is that the
automation object specifies a branch to use (or an empty value,
meaning let it default).
Michael Bridgen