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
$ hg init
$ touch a
$ unset HGUSER
$ echo "[ui]" >> .hg/hgrc
$ echo "username= foo" >> .hg/hgrc
$ echo " bar1" >> .hg/hgrc
$ hg ci -Am m
adding a
abort: username 'foo\nbar1' contains a newline
[255]
$ rm .hg/hgrc
$ HGUSER=`(echo foo; echo bar2)` hg ci -Am m
adding a
abort: username 'foo\nbar2' contains a newline
[255]
$ hg ci -Am m -u "`(echo foo; echo bar3)`"
adding a
transaction abort!
rollback completed
abort: username 'foo\nbar3' contains a newline
[255]