rewriteutil: add utility function to check if we can create new unstable cset

This patch adds a new file which will contain utility functions related to
rewritting changesets. It also adds a utility function to check if the
rewritting operation creates new unstable changesets and are we allowed to
create them.

This rewriteutil.py introduced in this patch and the utility functions added in
the upcoming patches exists in the evolve extension are being ported from there.

Differential Revision: https://phab.mercurial-scm.org/D1502

mercurial/rewriteutil.py

@@ -0,0 +1,25 b''
+# rewriteutil.py - utility functions for rewriting changesets
+#
+# Copyright 2017 Octobus <[e-mail]>
+#
+# 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 (
+    obsolete,
+    revset,
+)
+
+def disallowednewunstable(repo, revs):
+    """Checks whether editing the revs will create new unstable changesets and
+    are we allowed to create them.
+
+    To allow new unstable changesets, set the config:
+        `experimental.evolution.allowunstable=True`
+    """
+    allowunstable = obsolete.isenabled(repo, obsolete.allowunstableopt)
+    if allowunstable:
+        return revset.baseset()
+    return repo.revs("(%ld::) - %ld", revs, revs)