fix missing loop check

We failed to detect cyclic dependencies, leading to lost thread
roots.  In this Ruby version, I also screwed up the `seen' hash
check in a misguided effort to avoid an extra hash lookup.

The same algorithm was recently fixed in public-inbox:

  https://public-inbox.org/meta/20180425085249.14974-1-e@80x24.org/

Summarizing what happened with public-inbox: this algorithm can
still be thrown off when the References: order presented to us
is wrong, so sorting messages by age before feeding to
Msgthr#add can improve the results.

Regardless of ordering, messages should not become "lost" by the
algorithm.

1 files changed, 3 insertions, 2 deletions

diff --git a/lib/msgthr/container.rb b/lib/msgthr/container.rb
index fbff719..256033b 100644
--- a/lib/msgthr/container.rb
+++ b/lib/msgthr/container.rb
@@ -64,9 +64,10 @@ class Msgthr::Container
   end
 
   def has_descendent(child) # :nodoc:
-    seen = Hash.new(0)
+    seen = {}
     while child
-      return true if self == child || (seen[child] += 1) != 0
+      return true if self == child || seen[child]
+      seen[child] = true
       child = child.parent
     end
     false