revset: reintroduce and experimental revset for update destination

The revset is not ready for prime time yet. However it is useful to have some
version of it exposed to help candidate users to play with it and provide
feedback on what we should aim at.

We add a small test to make sure the code runs.

mercurial/revset.py

@@ -12,6 +12,7 b' import re'
 
 from .i18n import _
 from . import (
+    destutil,
     encoding,
     error,
     hbisect,
@@ -463,6 +464,11 b' def func(repo, subset, a, b):'
 
 # functions
 
+def _destupdate(repo, subset, x):
+    # experimental revset for update destination
+    args = getargsdict(x, 'limit', 'clean check')
+    return subset & baseset([destutil.destupdate(repo, **args)[0]])
+
 def _mergedefaultdest(repo, subset, x):
     # ``_mergedefaultdest()``
 
@@ -2142,6 +2148,7 b' def _hexlist(repo, subset, x):'
     return baseset([r for r in ls if r in s])
 
 symbols = {
+    "_destupdate": _destupdate,
     "_mergedefaultdest": _mergedefaultdest,
     "adds": adds,
     "all": getall,

tests/test-update-branches.t

@@ -260,3 +260,8 b' non-obsolete parent but that will be dec'
   $ hg debugobsolete bd10386d478cd5a9faf2e604114c8e6da62d3889
   $ hg up
   0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+Test experimental revset support
+
+  $ hg log -r '_destupdate()'
+  2:bd10386d478c 2 (no-eol)