Test that empty EXEPATH doesn't trigger the optimization

The `EXEPATH` optimization in `system_prefix()` on Windows relies
on `EXEPATH` either being unset or set to a value where it is okay
to attempt to find and use `clangarm64`, `mingw64`, and `mingw32`
subdirectories. Like most environment variables, `EXEPATH` is
unlikely to be directly controllable by an attacker. But it may
exist with a different meaning from any we (and Git for Windows)
intend. It also may inadvertently be set to an empty string, or
intnetionally set to a relative path; in either case, we cannot
safely use it for the `EXEPATH` optimization in `system_prefix()`.
In particular, if it is an empty string, then an attempt would be
made to use a `clangarm64`, `mingw64`, or `mingw32` subdirectory of
the current working directory.

This test will fail until the `EXEPATH` optimization is refined to
bail out--and fall back to the more robust strategy--if `EXEPATH`
is an empty string, or under broader conditions such as it not
being an absolute path.

Author: EliahKagan

gix-path/src/env/tests.rs

@@ -1,6 +1,7 @@
 mod system_prefix {
     use super::super::system_prefix_from_exepath_var;
     use gix_testtools::tempfile;
+    use serial_test::serial;
     use std::{ffi::OsString, path::PathBuf};
 
     struct ExePath {
@@ -104,4 +105,19 @@ mod system_prefix {
             assert_eq!(outcome, None);
         }
     }
+
+    #[test]
+    #[serial]
+    fn exepath_empty_string() {
+        for name in ["mingw32", "mingw64", "clangarm64"] {
+            let exepath = ExePath::new();
+            exepath.create_subdir(name);
+            let _cwd = gix_testtools::set_current_dir(&exepath.path).expect("can change to test dir");
+            let outcome = system_prefix_from_exepath_var(|key| match key {
+                "EXEPATH" => Some("".into()),
+                _ => None,
+            });
+            assert_eq!(outcome, None);
+        }
+    }
 }