Another issue is people working on monorepos (often backed by Git) 
sometimes want to split them into separate projects.

Either because they only work on a certain part of the whole project, or 
because they want to use Eglot, and that package has tied LSP roots 
detection to project roots (but the language server might expect some 
project configuration files in the root which reside in a subdirectory).

Here's a typical question/request for this functionality: 
https://emacs.stackexchange.com/questions/58463/project-el-override-project-root-with-dir-local-var/58468

Patch attached, it adds new user option project-vc-subprojects which 
alters the root-finding logic and cuts out the subproject contents from 
the parent project with the mechanism of "ignores".

The latter capability (excluding subproject's files) informed the choice 
of the approach. Which is altering the vc project backend's behavior, 
rather that offering this feature through another backend, like the one 
added in the previous patch, for example.

I don't know if all users of this feature will want them excluded, 
though. The attached implementation does, and maybe another option could 
be added to disable this.

Or we could drop this part of the behavior, insisting that users who 
want it could add the corresponding entries to project-vc-ignores. This 
way they would be listing the subprojects twice, however. And the 
project-vc-merge-submodules=nil behavior matches the other option 
(submodule files are excluded from the parent).

Again, thoughts welcome.