Avoid posting new diff when no changes in new commits

[jonnor]

Right now we just check that the whole range has not been analyzed before, and that there are any graph changes in the set.
This means that when adding several commits which does not change any graphs (on top of some previously diffet commits that did change graphs) - we post a whole diff. Would be better to treat this as a null/no-change.

[jonnor]

An example is here: imgflo/imgflo-server#86