fix: Prefer / in bash.exe path (for fixtures)

Starting in GitoxideLabs#1712, `gix-testtools` looks for `bash.exe` on Windows
in one of its common locations as provided by Git for Windows, by
taking the path given by `git --exec-path`, going up by three
components, and going down to `bin/bash.exe` under that. But the
`bin` and `bash.exe` components were appended in a way that used
`\` directory separators.

Ordinarily that would be ideal, since `\` is the primary directory
separator on Windows. However:

- Because `bash` is a Bourne-style shell, the path used to invoke
  it appears as `$0` in some circumstances, such as when it is used
  to run a command with `-c` with no arguments after its operand.
  Ideally it will never be assumed to use `/` separators, and
  ideally it will never be used in a way that treats backslashes
  appearing in it as escape characters. But in practice this is
  sometimes hard to avoid.

- The prefix, which is part of the output of `git --exec-path`,
  already uses `/` separatros, except in the unusual case that it
  overridden to a value containing `\` by setting the
  `GIT_EXEC_PATH` environment. So we are usually mixing `/` and `\`
  separators if we append with `\`, and usually avoiding mixing
  them if we append with `/`. Mixing directory separators is
  permitted, but the resulting paths are often more confusing when
  they appear in messages, and more broadly combine the
  disadvantages of each form.

This also refactors for clarity, and adds new tests related to the
change.

Author: EliahKagan

tests/tools/src/lib.rs

@@ -653,20 +653,27 @@ fn configure_command<'a, I: IntoIterator<Item = S>, S: AsRef<OsStr>>(
 }
 
 fn bash_program() -> &'static Path {
-    if cfg!(windows) {
-        // TODO(deps): Once `gix_path::env::shell()` is available, maybe do `shell().parent()?.join("bash.exe")`
-        static GIT_BASH: Lazy<Option<PathBuf>> = Lazy::new(|| {
+    // TODO(deps): *Maybe* add something to `gix-path` like `env::shell()` that can be used to
+    //             find bash and, once the version `gix-testtools` depends on has it, use it.
+    static GIT_BASH: Lazy<PathBuf> = Lazy::new(|| {
+        if cfg!(windows) {
             GIT_CORE_DIR
-                .parent()?
-                .parent()?
-                .parent()
-                .map(|installation_dir| installation_dir.join("bin").join("bash.exe"))
+                .ancestors()
+                .nth(3)
+                .map(OsString::from)
+                .map(|mut raw_path| {
+                    // Go down to where `bash.exe` usually is. Keep using `/` separators (not `\`).
+                    raw_path.push("/bin/bash.exe");
+                    raw_path
+                })
+                .map(PathBuf::from)
                 .filter(|bash| bash.is_file())
-        });
-        GIT_BASH.as_deref().unwrap_or(Path::new("bash.exe"))
-    } else {
-        Path::new("bash")
-    }
+                .unwrap_or_else(|| "bash.exe".into())
+        } else {
+            "bash".into()
+        }
+    });
+    GIT_BASH.as_ref()
 }
 
 fn write_failure_marker(failure_marker: &Path) {
@@ -1059,4 +1066,32 @@ mod tests {
     fn bash_program_ok_for_platform() {
         assert_eq!(bash_program(), Path::new("bash"));
     }
+
+    #[test]
+    fn bash_program_unix_path() {
+        let path = bash_program()
+            .to_str()
+            .expect("This test depends on the bash path being valid Unicode");
+        assert!(
+            !path.contains('\\'),
+            "The path to bash should have no backslashes, barring very unusual environments"
+        );
+    }
+
+    fn is_rooted_relative(path: impl AsRef<Path>) -> bool {
+        let p = path.as_ref();
+        p.is_relative() && p.has_root()
+    }
+
+    #[test]
+    #[cfg(windows)]
+    fn unix_style_absolute_is_rooted_relative() {
+        assert!(is_rooted_relative("/bin/bash"), "can detect paths like /bin/bash");
+    }
+
+    #[test]
+    fn bash_program_absolute_or_unrooted() {
+        let bash = bash_program();
+        assert!(!is_rooted_relative(bash), "{bash:?}");
+    }
 }