Fix Gitlab::Git::Repository#log_by_shell limit issue

What does this MR do?

Only append --max-count=#{limit} if limit != 0

Why was this MR needed?

It blows up in Gitaly otherwise for any requests that doesn't explicitly set limit https://gitlab.com/gitlab-org/gitaly/-/jobs/49303620

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

/label Gitaly /assign @jacobvosmaer-gitlab