When parsing `'xroot(a;-b)'` we were incorrectly returning
'-xroot(a;b)'` because we were treating `-` specially and before
treating parentheses or arguments.
Fixes: #959 (first comment)
Reported-by: mahi2003@users.github.com
Signed-off-by: Christophe de Dinechin <christophe@dinechin.org>