* Add checkout --merge-in-base support
When using this option, the checkout will try to merge in the
base branch into each module by running:
`git merge --no-edit origin/<base-branch>`
Retain the unmodified "BaseRevision" info in the Module struct
so it can be used in case it was overwritten by
`applyPullRequestsOverride()`
* only merge base if necessary
* Check if baseRevision is set
Co-authored-by: Bernd Ahlers <bernd@users.noreply.github.com>
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Co-authored-by: Bernd Ahlers <bernd@users.noreply.github.com>