rhg: Add support for the HGRCSKIPREPO environment variable

It’s easy enough and affects tests, well, that test that feature.

Differential Revision: https://phab.mercurial-scm.org/D10139

rust/hg-core/src/config/config.rs

@@ -24,6 +24,7 b' use crate::errors::{HgResultExt, IoResul'
 
 /// Holds the config values for the current repository
 /// TODO update this docstring once we support more sources
+#[derive(Clone)]
 pub struct Config {
     layers: Vec<layer::ConfigLayer>,
 }

rust/hg-core/src/repo.rs

@@ -169,7 +169,11 b' impl Repo {'
             reqs.extend(requirements::load(Vfs { base: &store_path })?);
         }
 
-        let repo_config = config.combine_with_repo(&repo_config_files)?;
+        let repo_config = if std::env::var_os("HGRCSKIPREPO").is_none() {
+            config.combine_with_repo(&repo_config_files)?
+        } else {
+            config.clone()
+        };
 
         let repo = Self {
             requirements: reqs,