stack: add a new module for stack-related commands

Bootstrap the module with a function that returns the stack for the current
revision. The definition of the stack is ihnerited from histedit default
revset and used by hg show stack.

Differential Revision: https://phab.mercurial-scm.org/D2395

mercurial/stack.py

@@ -0,0 +1,27 b''
+# stack.py - Mercurial functions for stack definition
+#
+#  Copyright Matt Mackall <[e-mail]> and other
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+from __future__ import absolute_import
+
+from . import (
+    revsetlang,
+    scmutil,
+)
+
+def getstack(repo, rev=None):
+    """return a smartrev of the stack containing either rev if it is not None
+    or the current working directory parent.
+
+    The stack will always contain all drafts changesets which are ancestors to
+    the revision and are not merges.
+    """
+    if rev is None:
+        rev = '.'
+
+    revspec = 'reverse(only(%s) and not public() and not ::merge())'
+    revset = revsetlang.formatspec(revspec, rev)
+    return scmutil.revrange(repo, [revset])