Declare that two "not" keywords cancel each other out

"not not return 34" exits with 34, not 1.  This behavior is pretty
surprising but benign. I think it's very unlikely that anyone relies
on the opposite behavior, because using two "not" decorators in one
job is weird, and code that compares not's raw exit code is rare.

The behavior doesn't match our docs, but it's not worth changing the
docs because that would confuse newcomers. Add a test to cement the
behavior and a comment to explain this is intentional.

I considered adding the comment at
parse_execution_context_t::populate_not_process where this behavior
is implemented but the field defintion seems even better, because I
expect programmers to read that first.

Closes #8377

src/proc.h

@@ -403,6 +403,7 @@ class job_t : noncopyable_t {
         bool notified_of_stop{false};
 
         /// Whether the exit status should be negated. This flag can only be set by the not builtin.
+        /// Two "not" prefixes on a single job cancel each other out.
         bool negate{false};
 
         /// This job is disowned, and should be removed from the active jobs list.

tests/checks/not.fish

@@ -0,0 +1,10 @@
+#RUN: %fish %s
+
+not true
+echo $status
+# CHECK: 1
+
+# Two "not" prefixes on a single job cancel each other out.
+not not sh -c 'exit 34'
+echo $status
+# CHECK: 34