show: use the new stack definition for show stack

Now that we have a common stack definition, use it in the hg show stack
command.

Differential Revision: https://phab.mercurial-scm.org/D2397

mercurial/destutil.py

@@ -13,6 +13,7 b' from . import ('
     error,
     obsutil,
     scmutil,
+    stack
 )
 
 def _destupdateobs(repo, clean):
@@ -355,9 +356,7 b' def desthistedit(ui, repo):'
     return None
 
 def stackbase(ui, repo):
-    # The histedit default base stops at public changesets, branchpoints,
-    # and merges, which is exactly what we want for a stack.
-    revs = scmutil.revrange(repo, [histeditdefaultrevset])
+    revs = stack.getstack(repo)
     return revs.last() if revs else None
 
 def _statusotherbook(ui, repo):