Add clarification about hash codes of 0.
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MOS_MIGRATED_REVID=100285082
diff --git a/src/main/java/com/google/devtools/build/skyframe/SkyKey.java b/src/main/java/com/google/devtools/build/skyframe/SkyKey.java
index d297c52..6916e66 100644
--- a/src/main/java/com/google/devtools/build/skyframe/SkyKey.java
+++ b/src/main/java/com/google/devtools/build/skyframe/SkyKey.java
@@ -67,7 +67,8 @@
// things going on here:
//
// (1) We use a value of 0 to indicate that the hash code hasn't been computed and cached yet.
- // Yes, this means that if the hash code is really 0 then we will "recompute" it each time.
+ // Yes, this means that if the hash code is really 0 then we will "recompute" it each time. But
+ // this isn't a problem in practice since a hash code of 0 should be rare.
//
// (2) Since we have no synchronization, multiple threads can race here thinking there are the
// first one to compute and cache the hash code.
