This reverts the GC behavior to the Go default of triggering GC when the
heap has grown 100% compared to after the previous GC. We were setting
this to 25% to keep memory usage at a minimum, but it has a pretty
severe performance cost (especially when syncing large files) as we keep
triggering GC too often.
This documents the tweak in the `-help` message so users can decide for
themselves, and sticks to whatever the Go runtime developers thinks is
best for the default.
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