Emacs: Eglot and darcs projects.

Playing around with eglot in projects managed by darcs I noticed that it would start a new language server for each new file opened instead of re-using the existing connection.

The cause seemed to be that it did not find the “project root” and thus handling each file as an own project.

Fixing it is simply done by running customize on the option project-vc-extra-root-markers and adding an entry _darcs to it. :-)