[nomerge] fix performance regression in mutable.HashMap#getOrElseUpdate

the change in question originated in
scala/collection-strawman#484. it was correct
at the time because `HashTable#addEntry0` has a threshold check

but then when the change was backported to 2.12.x in
scala/scala#6828, the
`HashTable#addEntry0`call was replaced with a call to
`HashMap#addEntry0`, which doesn't check the threshold. so if the
table is only ever updated using `getOrElseUpdate`, the table's load
factor would just keep climbing, resulting in poor performance

this was caught by my Project Euler solutions :-)

[nomerge] since the problem is specific to the 2.12 code

Author: SethTisue

library/src/scala/collection/mutable/HashMap.scala

@@ -87,7 +87,7 @@ extends AbstractMap[A, B]
       // Repeat search
       // because evaluation of `default` can bring entry with `key`
       val secondEntry = findEntry(key, newEntryIndex)
-      if (secondEntry == null) addEntry0(e, newEntryIndex)
+      if (secondEntry == null) addEntry(e, newEntryIndex)
       else secondEntry.value = default
       default
     }