phab-refresh: do not error out when the stack is empty

Actually, empty stack is easier to get than I expected (and harmless). When a
stack is publish, heptapod will detect the even, empty the stack (triggering CI)
and "forgetting" the branch afterward.

So we stop returning as error in this case.

Differential Revision: https://phab.mercurial-scm.org/D9513

contrib/phab-refresh-stack.sh

@@ -6,7 +6,7 b" revision_on_phab=`hg log --rev '.#stack "
 
 if [[ $revision_in_stack -eq 0 ]]; then
     echo "stack is empty" >&2
-    exit 1
+    exit 0
 fi
 
 if [[ $revision_on_phab -eq 0 ]]; then