Make `flux bootstrap` commit messages descriptive

[hiddeco]

At present, running flux bootstrap will always commit with the same commit message: Add manifests. This does not tell the user much about the change in the commit when they are inspecting the Git repository using e.g. git log.

The commit message should be changed to something that includes the version, or maybe even include if it was an install or an upgrade.

In addition, we may want to list the versions of all (changed) controllers in the long commit message.