feat: GIT_CONFIG_NOSYSTEM now also affects the installation directory.

It makes sense to consider it part of the 'system', and allows for proper
isolation of `gix` operations, for example in tests.

This is also a fix, as previously it checked for `...NO_SYSTEM`, instead of `NOSYSTEM`.

gix-config/src/source.rs

@@ -65,9 +65,15 @@ impl Source {
     pub fn storage_location(self, env_var: &mut dyn FnMut(&str) -> Option<OsString>) -> Option<Cow<'static, Path>> {
         use Source::*;
         match self {
-            GitInstallation => gix_path::env::installation_config().map(Into::into),
+            GitInstallation => {
+                if env_var("GIT_CONFIG_NOSYSTEM").is_some() {
+                    None
+                } else {
+                    gix_path::env::installation_config().map(Into::into)
+                }
+            }
             System => {
-                if env_var("GIT_CONFIG_NO_SYSTEM").is_some() {
+                if env_var("GIT_CONFIG_NOSYSTEM").is_some() {
                     None
                 } else {
                     env_var("GIT_CONFIG_SYSTEM")

gix-config/tests/config.rs

@@ -2,4 +2,5 @@ pub use gix_testtools::Result;
 
 mod file;
 mod parse;
+mod source;
 mod value;

gix-config/tests/source/mod.rs

@@ -0,0 +1,64 @@
+use gix_config::Source;
+use std::path::Path;
+
+#[test]
+fn git_config_no_system() {
+    assert_eq!(
+        Source::GitInstallation.storage_location(&mut |name| {
+            assert_eq!(
+                name, "GIT_CONFIG_NOSYSTEM",
+                "it only checks this var, and if set, nothing else"
+            );
+            Some("1".into())
+        }),
+        None
+    );
+    assert_eq!(
+        Source::System.storage_location(&mut |name| {
+            assert_eq!(
+                name, "GIT_CONFIG_NOSYSTEM",
+                "it only checks this var, and if set, nothing else"
+            );
+            Some("1".into())
+        }),
+        None
+    );
+}
+
+#[test]
+fn git_config_system() {
+    assert_eq!(
+        Source::System
+            .storage_location(&mut |name| {
+                match name {
+                    "GIT_CONFIG_NOSYSTEM" => None,
+                    "GIT_CONFIG_SYSTEM" => Some("alternative".into()),
+                    unexpected => unreachable!("unexpected env var: {unexpected}"),
+                }
+            })
+            .expect("set")
+            .as_ref(),
+        Path::new("alternative"),
+        "we respect the system config variable for overrides"
+    );
+}
+
+#[test]
+fn git_config_global() {
+    for source in [Source::Git, Source::User] {
+        assert_eq!(
+            source
+                .storage_location(&mut |name| {
+                    assert_eq!(
+                        name, "GIT_CONFIG_GLOBAL",
+                        "it only checks this var, and if set, nothing else"
+                    );
+                    Some("alternative".into())
+                })
+                .expect("set")
+                .as_ref(),
+            Path::new("alternative"),
+            "we respect the global config variable for 'git' overrides"
+        );
+    }
+}