revset: add a `node` key for sorting

Sorting by node give an arbitrary, but stable order which is useful for
sampling.

mercurial/revset.py

@@ -2293,6 +2293,7 b' def roots(repo, subset, x):'
     b'user': lambda c: c.user(),
     b'author': lambda c: c.user(),
     b'date': lambda c: c.date()[0],
+    b'node': lambda c: c.node(),
 }
 
 
@@ -2356,6 +2357,7 b' def sort(repo, subset, x, order):'
     - ``user`` for user name (``author`` can be used as an alias),
     - ``date`` for the commit date
     - ``topo`` for a reverse topographical sort
+    - ``node`` the nodeid of the revision
 
     The ``topo`` sort order cannot be combined with other sort keys. This sort
     takes one optional argument, ``topo.firstbranch``, which takes a revset that

tests/test-revset.t

@@ -2885,7 +2885,7 b' test sorting by multiple keys including '
   3 b112 m111 u11  120 0
   0 b12  m111 u112 111 10800
 
-  $ hg log -r 'sort(all(), "-desc -date")'
+  $ hg log -r 'sort(all(), "-desc -node")'
   1 b11  m12  u111 112 7200
   4 b111 m112 u111 110 14400
   3 b112 m111 u11  120 0