repo-info: declare the repo-info command

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Currently, git rev-parse covers a wide range of functionality not directly related to parsing revisions, as its name says. Over time, many features like parsing datestrings, options, paths, and others were added to it because there wasn't a more appropriated command to place them.

Create a new Git subcommand called repo-info. git repo-info will bring the functionality of retrieving repository-related information currently returned by rev-parse, displaying them in machine-readable formats (JSON or a key=value linewise format).