evolve: use stack alias s# in `hg evolve` msgs
As repository grows revision number lose their usefulness. To
make it a little better than now, after this patch we will be
using short stack aliases introduced by the topic extension
(as displayed by `hg stack`).
These stack aliases s# will be used only when user has enabled
topic extension and his repo's current active topic is same as
the revs to be evolved.
For now, this patch doesn't cover `continue` case. I will discuss about this
with Pierre-Yves David. Maybe we have to store the info in evolvestate that "we
were using stack aliases".