posix: quote the specified string only when it may have to be quoted This patch makes "posix.shellquote" examine the specified string and quote it only when it may have to be quoted for safety, like as the previous patch for "windows.shellquote". In fact, on POSIX environment, quoting itself doesn't cause issues like issue4463. But (almost) equivalent quoting policy can avoid examining test result differently on POSIX and Windows (even though showing command line with "%r" causes such examination in "test-extdiff.t"). The last hunk for "test-extdiff.t" in this patch isn't needed for the previous patch for "windows.shellquote", because the code path of it is executed only "#if execbit" (= avoided on Windows).