fix: editor var expansion (#603)

This handles cases where the git editor is set to `$EDITOR`. The steps for reproducing are as follows.

1. Set the EDITOR env var
2. Update your .gitconfig to include editor=$EDITOR
3. Run `av pr --edit`

According to `man 1 git-var`, `GIT_EDITOR` is "meant to be interpreted by the shell when it is used". Hence, this variable needs to be interpreted.

Credit to @tgandrews for spotting the issue

Author: Brookke

internal/editor/editor.go

@@ -69,16 +69,12 @@ func Launch(ctx context.Context, repo *git.Repo, config Config) (string, error)
 	}
 
 	// Launch the editor as a subprocess.
-	// We interpret the command with shell syntax to allow users to specify
-	// both flags and use editor executables with spaces.
+	// We execute the command through a shell to properly handle environment
+	// variable expansion and shell quoting rules, matching Git's behavior.
 	// e.g., EDITOR="'/path/with spaces/editor'" or
-	// EDITOR="code --wait" work.
-	args, err := shellquote.Split(config.Command)
-	if err != nil {
-		return "", errors.Wrapf(err, "invalid editor command: %q", config.Command)
-	}
-	args = append(args, tmp.Name())
-	cmd := exec.CommandContext(ctx, args[0], args[1:]...)
+	// EDITOR="code --wait" or GIT_EDITOR="$EDITOR" work correctly.
+	shellCmd := config.Command + " " + shellquote.Join(tmp.Name())
+	cmd := exec.CommandContext(ctx, "sh", "-c", shellCmd)
 	cmd.Stdin = os.Stdin
 	cmd.Stdout = os.Stdout
 	stderr := bytes.NewBuffer(nil)