Merge external custom reasons

Author: konstin

src/report.rs

@@ -144,6 +144,9 @@ impl<P: Package, VS: VersionSet, M: Eq + Clone + Debug + Display> DerivationTree
             //
             // Cannot be merged because the reason may not match
             DerivationTree::External(External::NoVersions(_, _)) => None,
+            DerivationTree::External(External::Custom(_, r, reason)) => Some(
+                DerivationTree::External(External::Custom(package, set.union(&r), reason)),
+            ),
             DerivationTree::External(External::FromDependencyOf(p1, r1, p2, r2)) => {
                 if p1 == package {
                     Some(DerivationTree::External(External::FromDependencyOf(
@@ -161,8 +164,6 @@ impl<P: Package, VS: VersionSet, M: Eq + Clone + Debug + Display> DerivationTree
                     )))
                 }
             }
-            // Cannot be merged because the reason may not match
-            DerivationTree::External(External::Custom(_, _, _)) => None,
         }
     }
 }