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

@@ -1,34 +1,34 b''
 #!/bin/bash
 set -eu
 
 revision_in_stack=`hg log --rev '.#stack and ::.' -T '\nONE-REV\n' | grep 'ONE-REV' | wc -l`
 revision_on_phab=`hg log --rev '.#stack and ::. and desc("re:\nDifferential Revision: [^\n]+D\d+$")' -T '\nONE-REV\n' | grep 'ONE-REV' | wc -l`
 
 if [[ $revision_in_stack -eq 0 ]]; then
     echo "stack is empty" >&2
-    exit 1
+    exit 0
 fi
 
 if [[ $revision_on_phab -eq 0 ]]; then
     echo "no tracked diff in this stack" >&2
     exit 0
 fi
 
 if [[ $revision_on_phab -lt $revision_in_stack ]]; then
     echo "not all stack changesets (${revision_in_stack}) have matching Phabricator Diff (${revision_on_phab})" >&2
     exit 2
 fi
 
 if [[ "$PHABRICATOR_TOKEN" == "" ]]; then
     echo 'missing $PHABRICATOR_TOKEN variable' >&2
     exit 2
 fi
 
 hg \
 --config extensions.phabricator= \
 --config phabricator.url=https://phab.mercurial-scm.org/ \
 --config phabricator.callsign=HG \
 --config auth.phabricator.schemes=https \
 --config auth.phabricator.prefix=phab.mercurial-scm.org \
 --config auth.phabricator.phabtoken=$PHABRICATOR_TOKEN \
 phabsend --rev '.#stack and ::.'