Simplify path construction and config; improve comments

This stops attempting to make the Windows-specific helper function
semi-portable. I had done that with the idea tha it could be tested
on all platforms so as to gain better coverage in practice. But it
doesn't really make sense to do this because it keeps the native
path separator `\` from being used even where it is now clearer to
use it, and because such tests would not be able to use common
values such as `C:\Program Files` because a path that starts with
`C:\`, while absolute on Windows, is relative on Unix-like systems.

Author: EliahKagan

gix-path/src/env/git.rs

@@ -15,7 +15,7 @@ pub(super) static ALTERNATIVE_LOCATIONS: Lazy<Vec<PathBuf>> = Lazy::new(|| {
 #[cfg(not(windows))]
 pub(super) static ALTERNATIVE_LOCATIONS: Lazy<Vec<PathBuf>> = Lazy::new(|| vec![]);
 
-#[cfg(any(windows, test))]
+#[cfg(windows)]
 fn alternative_windows_locations_from_environment<F>(var_os_func: F) -> Vec<PathBuf>
 where
     F: Fn(&str) -> Option<std::ffi::OsString>,
@@ -38,10 +38,10 @@ where
     let varname_current = "ProgramFiles";
 
     // 64-bit relative bin dir. So far, this is always mingw64, not ucrt64, clang64, or clangarm64.
-    let suffix_64 = Path::new("Git/bin/mingw64");
+    let suffix_64 = Path::new(r"Git\bin\mingw64");
 
     // 32-bit relative bin dir. So far, this is always mingw32, not clang32.
-    let suffix_32 = Path::new("Git/bin/mingw32");
+    let suffix_32 = Path::new(r"Git\bin\mingw32");
 
     // Whichever of the 64-bit or 32-bit relative bin better matches this process's architecture.
     // Unlike the system architecture, the process architecture is always known at compile time.
@@ -62,12 +62,11 @@ where
         if let Some(value) = var_os_func(name) {
             let pf = Path::new(&value);
             if pf.is_relative() {
-                // This should never happen, but if it does then we should not use the path.
+                // This shouldn't happen, but if it does then don't use the path. This is mainly in
+                // case we are accidentally invoked with the environment variable set but empty.
                 continue;
             };
-            // Chain components to weakly normalize the path, mostly just for its separators.
-            let components = pf.iter().chain(suffix.iter());
-            let location = PathBuf::from_iter(components);
+            let location = pf.join(suffix);
             if !locations.contains(&location) {
                 locations.push(location);
             }
@@ -289,7 +288,7 @@ mod tests {
                 .expect("The x86 program files folder will in practice always be available.");
 
             let maybe_pf_64bit = RegKey::predef(HKEY_LOCAL_MACHINE)
-                .open_subkey_with_flags(r#"SOFTWARE\Microsoft\Windows\CurrentVersion"#, KEY_QUERY_VALUE)
+                .open_subkey_with_flags(r"SOFTWARE\Microsoft\Windows\CurrentVersion", KEY_QUERY_VALUE)
                 .expect("The `CurrentVersion` key exists and allows reading.")
                 .get_value::<OsString, _>("ProgramW6432Dir")
                 .map(PathBuf::from)