Make while loops evaluate to the last executed command status

A while loop now evaluates to the last executed command in the body, or
zero if the loop body is empty. This matches POSIX semantics.

Add a bunch of tricky tests.

See #4982

tests/while.in

@@ -18,10 +18,56 @@ function runs_once
 end
 
 # this should return 1
-never_runs; echo $status
+never_runs; echo "Empty Loop in Function: $status"
 
 # this should return 0
-runs_once; echo $status
+runs_once; echo "Runs Once: $status"
 
 # this should return 2
-early_return; echo $status
+early_return; echo "Early Return: $status"
+
+logmsg Loops exit status handling
+
+function set_status ; return $argv[1]; end
+
+# The previous status is visible in the loop condition.
+# This includes both the incoming status, and the last command in the
+# loop body.
+set_status 36
+while begin
+     set -l saved $status
+     echo "Condition Status: $status"
+     set_status $saved
+  end
+  true
+end
+
+# The condition status IS visible in the loop body.
+set_status 55
+while true
+  echo "Body Status: $status"
+  break
+end
+
+# The status of the last command is visible in the loop condition
+set_status 13
+while begin
+     set -l saved $status
+     echo "Condition 2 Status: $saved"
+     test $saved -ne 5
+  end
+  set_status 5
+end
+
+# The status of the last command is visible outside the loop
+set rem 5 7 11
+while [ (count $rem) -gt 0 ]
+  set_status $rem[1]
+  set rem $rem[2..-1]
+end
+echo "Loop Exit Status: $status"
+
+# Empty loops succeed.
+false
+while false; end
+echo "Empty Loop Status: $status"