mathlib3

1278efd6 - Fix `tactic.exact` timeout in `apply'` (#1371)

Fix `tactic.exact` timeout in `apply'` (#1371) Sometimes `tactic.exact` may timeout for no reason. See zulip discussion https://leanprover.zulipchat.com/#narrow/stream/113489-new-members/topic/.60apply'.60timeout/near/174415043