gitleaks: limit the amount of produced output

[kdudka]

The fix for issue #98 was incomplete. Gitleaks is able to produce a few gigabytes of data even if we rate limit the number of warnings at 1024 (because it pastes the secrets as excessively long lines). This can be reproduced with nss-util-3.90.0-1.el7_9. We need make the gitleaks plug-in use csgrep --limit-msg-len once it is implemented (issue csutils/csdiff#133)