add some tests to be clearer about the kind of command that is created (#1103)

Note that for a moment, there was the idea about using `shell-words`
split certain invocations ourselves to avoid having to use a shell.

This is great on windows, where a shell might not be available.
On linux though this also means that some shell-builtins might
not be not be found if this is used, which limits the approach
to windows, and even there it might reduce compatibility.

Thus, let's not do it.

Author: Byron

gix-command/src/lib.rs

@@ -34,7 +34,8 @@ mod prepare {
 
     /// Builder
     impl Prepare {
-        /// If called, the command will not be executed directly, but with `sh`.
+        /// If called, the command will not be executed directly, but with `sh`, but ony if the
+        /// command passed to [`prepare`](super::prepare()) requires this.
         ///
         /// This also allows to pass shell scripts as command, or use commands that contain arguments which are subsequently
         /// parsed by `sh`.

gix-command/tests/command.rs

@@ -1,14 +1,65 @@
 use gix_testtools::Result;
 
 mod prepare {
+    #[cfg(windows)]
+    const SH: &str = "sh";
+    #[cfg(not(windows))]
+    const SH: &str = "/bin/sh";
+
     fn quoted(input: &[&str]) -> String {
         input.iter().map(|s| format!("\"{s}\"")).collect::<Vec<_>>().join(" ")
     }
+
+    #[test]
+    fn empty() {
+        let cmd = std::process::Command::from(gix_command::prepare(""));
+        assert_eq!(format!("{cmd:?}"), "\"\"");
+    }
     #[test]
     fn single_and_multiple_arguments() {
         let cmd = std::process::Command::from(gix_command::prepare("ls").arg("first").args(["second", "third"]));
         assert_eq!(format!("{cmd:?}"), quoted(&["ls", "first", "second", "third"]));
     }
+
+    #[test]
+    fn single_and_multiple_arguments_as_part_of_command() {
+        let cmd = std::process::Command::from(gix_command::prepare("ls first second third"));
+        assert_eq!(
+            format!("{cmd:?}"),
+            quoted(&["ls first second third"]),
+            "without shell, this is an invalid command"
+        );
+    }
+
+    #[test]
+    fn single_and_multiple_arguments_as_part_of_command_with_shell() {
+        let cmd = std::process::Command::from(gix_command::prepare("ls first second third").with_shell());
+        assert_eq!(
+            format!("{cmd:?}"),
+            quoted(&[SH, "-c", "ls first second third", "--"]),
+            "with shell, this works as it performs word splitting"
+        );
+    }
+
+    #[test]
+    fn single_and_complex_arguments_as_part_of_command_with_shell() {
+        let cmd = std::process::Command::from(gix_command::prepare("ls --foo \"a b\"").arg("additional").with_shell());
+        assert_eq!(
+            format!("{cmd:?}"),
+            format!("\"{SH}\" \"-c\" \"ls --foo \\\"a b\\\" \\\"$@\\\"\" \"--\" \"additional\""),
+            "with shell, this works as it performs word splitting"
+        );
+    }
+
+    #[test]
+    fn tilde_path_and_multiple_arguments_as_part_of_command_with_shell() {
+        let cmd = std::process::Command::from(gix_command::prepare("~/bin/exe --foo \"a b\"").with_shell());
+        assert_eq!(
+            format!("{cmd:?}"),
+            format!("\"{SH}\" \"-c\" \"~/bin/exe --foo \\\"a b\\\"\" \"--\""),
+            "this always needs a shell as we need tilde expansion"
+        );
+    }
 }
 
 mod spawn {