update_pushes_since_gc may not be reset to 0 and cause many `git gc` instances to run
d91c6c07 changed it so that the project
pushes_since_gc will only be changed at most once per 60 seconds. Here's where that could cause problems:
git pushhappens, triggering the
git gcruns, clearing out the pack file
git pushoccurs, but since the value was just incremented, we can't reset the counter and repeat step 1.
I'm wondering if we always want to allow the counter to go to 0.