Believe it or not, the memory these references hold alive is significant.
Nulling them out saves 5-10K per process.
MozReview-Commit-ID: JONjE48yE8I
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extra : rebase_source : 49adc538070eecb9183e6e052e6e43db9c4c7a99
extra : histedit_source : 699f49fad1bfa69b8c511bf96187096e751c8606