Fix chart with deleted builds

[il--ya]

When builds are deleted, there are gaps in build numbers, and chart doesn't show anything before deleted build. This causes JENKINS-52711.
I haven't tested this fix, as I am stuck at the moment with out-of-date Jenkins version, but I think it should work.

[il--ya]

ping @jglick @dwnusbaum

[jglick]

Maybe? Likely possible to write a test for this using RunLoadCounter.

[il--ya]

I don't think tests update is necessary for such a simple bug fix. And I don't have capacity to do that anyway. @jglick do you see any particular reason for concern with this change?

[zbynek]

@il--ya are you sure this won't load additional builds? Like if loaded builds contains 1 and 999, this may load builds 998, 997, ...

Even though I'm not sure how realistic that scenario is, it may be safer to put loaded builds to a TreeSet and get the previous build as loadedBuildsTreeSet == null ? current - 1 : loadedBuildsTreeSet.lower(current)

[jglick]

do you see any particular reason for concern with this change?

Having written the condition being patched, yes I do. It is not obvious to me that this is right, especially if you say you have not tested it interactively either. This is a very widely used plugin, so any regression would likely be worse than the reported limitation. Would need to spend some time refamiliarizing myself with this.