gerrit/update-repos: Set up operations/mediawiki-config like production

We use a standard project.config for all repos imported into train-dev's Gerrit.

This commit adds the following additional project.config when importing the operations/mediawiki-config repo for the first time. Note that this change does not affect the repo if it already exists.

[submit]
	mergeContent = false
	action = rebase if necessary

This is the same configuration as production and is important for testing T343967.