Reset project pushes_since_gc when we enqueue the git gc call

What does this MR do?

Reset counters just when we really enqueue the git gc command. So as this is part of the #executemethod we throttle the enqueue and the reset (database statement) as we throttle the #increment! call. And we only call #execute if it's needed.

What are the relevant issue numbers?

Closes #19711 (closed)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together