rust-discovery: optimization of add commons/missings for empty arguments

These two cases have to be catched early for different reasons.
In the case of add_missing_revisions, we don't want to trigger
the computation of the undecided set (and the children cache)
too early: the later the better.

In the case of add_common_revisions, the inner `MissingAncestors`
object wouldn't know that all ancestors of its bases have already
been removed from the undecided.
In principle, that would in itself be a lead for further
improvement: this remove_ancestors_from could be more incremental,
but the current performance seems to be good enough.

Differential Revision: https://phab.mercurial-scm.org/D6429

rust/hg-core/src/discovery.rs

@@ -225,7 +225,11 b' impl<G: Graph + Clone> PartialDiscovery<'
         &mut self,
         common: impl IntoIterator<Item = Revision>,
     ) -> Result<(), GraphError> {
+        let before_len = self.common.get_bases().len();
         self.common.add_bases(common);
+        if self.common.get_bases().len() == before_len {
+            return Ok(());
+        }
         if let Some(ref mut undecided) = self.undecided {
             self.common.remove_ancestors_from(undecided)?;
         }
@@ -246,11 +250,14 b' impl<G: Graph + Clone> PartialDiscovery<'
         &mut self,
         missing: impl IntoIterator<Item = Revision>,
     ) -> Result<(), GraphError> {
+        let mut tovisit: VecDeque<Revision> = missing.into_iter().collect();
+        if tovisit.is_empty() {
+            return Ok(());
+        }
         self.ensure_children_cache()?;
         self.ensure_undecided()?; // for safety of possible future refactors
         let children = self.children_cache.as_ref().unwrap();
         let mut seen: HashSet<Revision> = HashSet::new();
-        let mut tovisit: VecDeque<Revision> = missing.into_iter().collect();
         let undecided_mut = self.undecided.as_mut().unwrap();
         while let Some(rev) = tovisit.pop_front() {
             if !self.missing.insert(rev) {