Reform positional argument parsing (#523)

This diff makes positional argument parsing much cleaner, along with
adding a bunch of tests. Just's positional argument parsing is rather,
complex, so hopefully this reform allows it to both be correct and stay
correct.

User-visible changes:

- `just ..` is now accepted, with the same effect as `just ../`

- `just .` is also accepted, with the same effect as `just`

- It is now an error to pass arguments or overrides to subcommands
  that do not accept them, namely `--dump`, `--edit`, `--list`,
  `--show`, and `--summary`. It is also an error to pass arguments to
  `--evaluate`, although `--evaluate` does of course still accept
  overrides.

  (This is a breaking change, but hopefully worth it, as it will allow us
  to add arguments to subcommands which did not previously take
  them, if we so desire.)

- Subcommands which do not accept arguments may now accept a
  single search-directory argument, so `just --list ../` and
  `just --dump foo/` are now accepted, with the former starting the
  search for the justfile to list in the parent directory, and the latter
  starting the search for the justfile to dump in `foo`.

tests/readme.rs

@@ -0,0 +1,41 @@
+use std::{fs, process::Command};
+
+use executable_path::executable_path;
+use test_utilities::{assert_success, tempdir};
+
+#[test]
+fn readme() {
+  let mut justfiles = vec![];
+  let mut current = None;
+
+  for line in fs::read_to_string("README.adoc").unwrap().lines() {
+    if let Some(mut justfile) = current {
+      if line == "```" {
+        justfiles.push(justfile);
+        current = None;
+      } else {
+        justfile += line;
+        justfile += "\n";
+        current = Some(justfile);
+      }
+    } else if line == "```make" {
+      current = Some(String::new());
+    }
+  }
+
+  for justfile in justfiles {
+    let tmp = tempdir();
+
+    let path = tmp.path().join("justfile");
+
+    fs::write(&path, &justfile).unwrap();
+
+    let output = Command::new(executable_path("just"))
+      .current_dir(tmp.path())
+      .arg("--dump")
+      .output()
+      .unwrap();
+
+    assert_success(&output);
+  }
+}

tests/search.rs

@@ -121,3 +121,27 @@ fn test_downwards_multiple_path_argument() {
   search_test(&path, &["./a/b/"]);
   search_test(&path, &["./a/b/default"]);
 }
+
+#[test]
+fn single_downards() {
+  let tmp = tmptree! {
+    justfile: "default:\n\techo ok",
+    child: {},
+  };
+
+  let path = tmp.path();
+
+  search_test(&path, &["child/"]);
+}
+
+#[test]
+fn single_upwards() {
+  let tmp = tmptree! {
+    justfile: "default:\n\techo ok",
+    child: {},
+  };
+
+  let path = tmp.path().join("child");
+
+  search_test(&path, &["../"]);
+}