Render the time bounds with getutc so the caller's Time is left alone

[Thayorns]

Fbe::Graph rendered the time bounds of pull_requests_with_reviews, total_commits_pushed and total_issues_created with since.utc.iso8601. Time#utc converts the receiver itself, so these methods changed the caller's Time to UTC and raised FrozenError on a frozen one, as #1231 shows.

Now all four spots use getutc, which returns a new Time and leaves the argument alone. The query text is the same as before.

The tests stub query and check that a since or till with a random offset keeps it after the call, that a frozen Time is accepted by each of the three methods, and that the search window is still rendered in UTC.

Closes #1231

[Thayorns]

@yegor256 take a look please, happy to clarify anything about this change.