revset: change ancestor to accept 0 or more arguments (issue3750)

Change ancestor to accept 0 or more arguments. The greatest common ancestor of a
single changeset is that changeset. If passed no arguments, the empty list is
returned.

mercurial/revset.py

@@ -277,20 +277,32 b' def adds(repo, subset, x):'
     return checkstatus(repo, subset, pat, 1)
 
 def ancestor(repo, subset, x):
-    """``ancestor(single, single)``
-    Greatest common ancestor of the two changesets.
+    """``ancestor(*changeset)``
+    Greatest common ancestor of the changesets.
+
+    Accepts 0 or more changesets.
+    Will return empty list when passed no args.
+    Greatest common ancestor of a single changeset is that changeset.
     """
     # i18n: "ancestor" is a keyword
-    l = getargs(x, 2, 2, _("ancestor requires two arguments"))
-    r = list(repo)
-    a = getset(repo, r, l[0])
-    b = getset(repo, r, l[1])
-    if len(a) != 1 or len(b) != 1:
-        # i18n: "ancestor" is a keyword
-        raise error.ParseError(_("ancestor arguments must be single revisions"))
-    an = [repo[a[0]].ancestor(repo[b[0]]).rev()]
+    l = getlist(x)
+    rl = list(repo)
+    anc = None
 
-    return [r for r in an if r in subset]
+    # (getset(repo, rl, i) for i in l) generates a list of lists
+    rev = repo.changelog.rev
+    ancestor = repo.changelog.ancestor
+    node = repo.changelog.node
+    for revs in (getset(repo, rl, i) for i in l):
+        for r in revs:
+            if anc is None:
+                anc = r
+            else:
+                anc = rev(ancestor(node(anc), node(r)))
+
+    if anc is not None and anc in subset:
+        return [anc]
+    return []
 
 def _ancestors(repo, subset, x, followfirst=False):
     args = getset(repo, list(repo), x)

tests/test-revset.t

@@ -218,17 +218,29 b' quoting needed'
   $ log 'date(2005) and 1::'
   4
 
+ancestor can accept 0 or more arguments
+
+  $ log 'ancestor()'
   $ log 'ancestor(1)'
-  hg: parse error: ancestor requires two arguments
-  [255]
+  1
   $ log 'ancestor(4,5)'
   1
   $ log 'ancestor(4,5) and 4'
+  $ log 'ancestor(0,0,1,3)'
+  0
+  $ log 'ancestor(3,1,5,3,5,1)'
+  1
+  $ log 'ancestor(0,1,3,5)'
+  0
+  $ log 'ancestor(1,2,3,4,5)'
+  1
   $ log 'ancestors(5)'
   0
   1
   3
   5
+  $ log 'ancestor(ancestors(5))'
+  0
   $ log 'author(bob)'
   2
   $ log 'author("re:bob|test")'