revset: predicate to avoid lookup errors

A query like

head() and (descendants("bad") and not descendants("fix"))

(testing if repo heads are affected by a bug) will abort with a
RepoLookupError if either badrev or fixrev aren't found inside
the repository, which is not very informative.

The new predicate returns an empty set for lookup errors, so

head() and (descendants(present("bad")) and not descendants(present("fix")))

will behave as wanted even if those revisions are not found.

mercurial/help/revsets.txt

@@ -122,6 +122,10 b' The following predicates are supported:'
 ``parents(set)``
   The set of all parents for all changesets in set.
 
+``present(set)``
+  An empty set, if any revision in set isn't found; otherwise,
+  all revisions in set.
+
 ``removes(pattern)``
   Changesets which remove files matching pattern.
 

mercurial/revset.py

@@ -378,6 +378,12 b' def reverse(repo, subset, x):'
     l.reverse()
     return l
 
+def present(repo, subset, x):
+    try:
+        return getset(repo, subset, x)
+    except error.RepoLookupError:
+        return []
+
 def sort(repo, subset, x):
     l = getargs(x, 1, 2, _("sort wants one or two arguments"))
     keys = "rev"
@@ -481,6 +487,7 b' symbols = {'
     "p1": p1,
     "p2": p2,
     "parents": parents,
+    "present": present,
     "removes": removes,
     "reverse": reverse,
     "roots": roots,