It's not a problem of git, but a problem of abstraction and modularization, isn't it. In principle, this is partly doable with Wikidata (or old-school templates

It's not a problem of git, but a problem of abstraction and modularization, isn't it. In principle, this is partly doable with Wikidata (or old-school templates), e.g. some information in infoboxes and country flags are retrieved from a central location and by editing this single location, you'd change the relevant information on all pages simultaneously. However, use of such abstractions is fairly limited (especially usage of Wikidata on enwiki), basically country flags and similar are the only such usage, I'd say. In the future, this exact problem is one the primary goals of the Abstract Wikipedia project (see https://meta.m.wikimedia.org/wiki/Abstract_Wikipedia https://meta.m.wikimedia.org/wiki/Abstract_Wikipedia). But again, I don't think enwiki would use this very much, anyway.