Start testing that installation config is not from GIT_CONFIG

These tests should not pass yet, because we currently do allow
`GIT_CONFIG` to be read. Although the optimization that reads
`EXEPATH` will sometimes prevent it from being used, it will
usually be used, becuase `EXEPATH` is usually not set on Windows
(and outside of Windows, that optimization is inapplicable).
Furthermore, the tests in `gix-path/src/env/git/tests.rs` bypass
that optimization.

Two of the three new tests rightly fail. But one of them,
`never_from_git_config_env_var`, wrongly passes, due to a big in
the test, described in the comments there.

Author: EliahKagan

gix-path/src/env/git/tests.rs

@@ -453,6 +453,13 @@ mod exe_info {
         check_exe_info();
     }
 
+    #[test]
+    #[serial]
+    fn tolerates_git_config_env_var() {
+        let _env = gix_testtools::Env::new().set("GIT_CONFIG", NULL_DEVICE);
+        check_exe_info();
+    }
+
     #[test]
     #[serial]
     fn same_result_with_broken_temp() {
@@ -480,6 +487,19 @@ mod exe_info {
         assert_eq!(with_unmodified_env, with_oversanitized_env);
     }
 
+    #[test]
+    #[serial]
+    fn same_result_with_git_config_env_var() {
+        let with_unmodified_env = exe_info();
+
+        let with_git_config_env_var = {
+            let _env = gix_testtools::Env::new().set("GIT_CONFIG", NULL_DEVICE);
+            exe_info()
+        };
+
+        assert_eq!(with_unmodified_env, with_git_config_env_var);
+    }
+
     #[test]
     #[serial]
     #[cfg(not(target_os = "macos"))] // Assumes no higher "unknown" scope. The `nosystem` case works.
@@ -556,6 +576,34 @@ mod exe_info {
         );
     }
 
+    #[test]
+    #[serial]
+    fn never_from_git_config_env_var() {
+        let repo = gix_testtools::scripted_fixture_read_only("local_config.sh").expect("script succeeds");
+
+        // FIXME: exe_info() changes directory, so this is not seen, which results in this test
+        // wrongly passing even if exe_info() does not really ensure GIT_CONFIG is ignore. So
+        // either make this path an absolute non-UNC path (non-UNC to ensure all versions of Git
+        // accept it) or use a newly created temporary file rather than the fixture.
+        let config_path = repo
+            .join(".git")
+            .join("config")
+            .to_str()
+            .expect("valid UTF-8")
+            .to_owned();
+
+        let _env = gix_testtools::Env::new()
+            .set("GIT_CONFIG_NOSYSTEM", "1")
+            .set("GIT_CONFIG_GLOBAL", NULL_DEVICE)
+            .set("GIT_CONFIG", config_path);
+
+        let maybe_path = exe_info();
+        assert_eq!(
+            maybe_path, None,
+            "Should find no config path from GIT_CONFIG (even if nonempty)"
+        );
+    }
+
     #[test]
     fn first_file_from_config_with_origin() {
         let macos =