`GitUtils` determines the Discourse version information which is shown
in the user interface. This commit introduces an optional
`config/git-utils-overrides.json` file, which hosting providers can use
to override this information. For example, if the hosting provider is
needs to apply hosting-platform-specific patches or embargoed security
fixes on top of a normal release branch.
Previously there were no specs for this file. So this commit introduces
specs for the default behaviour, and the new overrides feature.