bug#41736: 28.0.50; project.el - support global patterns for project-ignores

[Theodor Thornhill]

Hello,

In https://debbugs.gnu.org/cgi/bugreport.cgi?bug=41572 there is talk about 
supporting a "plain project", and when joining that chat I realised that the 
"best" current implementation is the VC backend. However, this backend does not 
easily include other patterns to ignore.

For example we might want to ignore "node_modules", "elm-stuff" etc.
These can be added pretty easily to 'project-vc-ignores' via:

1. Major mode support: "(setq-local project-vc-ignores '("foo" "bar"))"
2. User init.el "(add-to-list project-vc-ignores "foo")"

However, there are some drawbacks.
Method 1:
- We rely on major mode to implement this
- When using "project-switch-project", then press "f - find-file" the ignore is 
not recognized yet since it relies on the major mode.
- It does not work when a project uses several modes

Method 2:
- Needs user to add this to own init file.
- Pollutes the defcustom globally

Is there a way to set these patters such that we still can use the fast VC 
backend, but not have these drawbacks?

It seems like an easy solution could be to add a "defcustom 
project-ignore-patterns" that the project-vc backend picks up. This could be 
set by both future major modes and as user customization.

FWIW, method 2 seems to work perfectly, yet seems like kind of a hack.

Theo