`brew bump` will check for PRs related to a package even if the
package version and livecheck version are the same. We're presumably
only interested in related PRs when the livecheck version differs, so
we can reduce GitHub API requests by skipping the check(s) when the
versions are equal. We use `bump` in `autobump` workflows, so this
should help with recent rate limiting issues (e.g., 3203 out of 3426
autobumped formulae were up to date in a recent run).
This also reworks the output for duplicate PRs, making it clear when
`bump` skipped checking PRs (as printing "none" would suggest that
PRs were checked) and only printing the "Maybe duplicate" information
when checked. This makes it a little easier to understand what `bump`
has done internally from the output.
