Repo "aragost/projrc" requires authorization although there are no changes to push (BB-4794)

I use aragost/projrc as a subrepo. I have no changes to be pushed in this subrepo (and I have no permission to push to that subrepo). When I try to push the master repo, projrc requests authorization. We have several other subrepos from bitbucket (for example, onsub), where this is not the case.

To reproduce the problem, just clone aragost/projrc, don't change anything and try to push:

{{{
D:\Users\schueler\tmp\projrc>hg push
pushing to https://bitbucket.org/aragost/projrc\\
searching for changes
no changes found
http authorization required
realm: Bitbucket.org HTTP
user: lantiq
password:
}}}

As far as I understand it, you should be able to push when there are no changes, because the discovery phase of push does only require read permission (which I obviously have, because I can clone the repo). See, for example, this discussion: http://mercurial.808500.n3.nabble.com/Read-Only-Subrepos-td1035083.html. I'm especially referring to this paragraph:

{{{
An empty push to a pullable repo should always work, regardless of
whether you have permission to push or not. The first part of the push,
discovery, is allowed for both push and pull, and the second part
(actually sending a bundle) is skipped if there's nothing to send.

So unmodified subrepos shouldn't be causing a problem.
}}}

I'm using the current Mercurial, version 2.2.3.