BUG? git log --no-merges shows grafted merges in shallow clones

> Hi,
>
> I don't think that's intended. When running "git log --no-merges" in a
> shallow clone, and the last commit in the history is a merge commit,
> "git log --no-merges" still shows it.
>
> I've just hit this in a test running on a --depth=50 clone on Travis-CI
> on git-multimail:
>
> $ git cat-file -p c3c1cc25b27d448e9ef67b265a11be8735ff2df4
> tree c341dd60c4b639eac1d6dcc3caffb5d7201c2245
> parent b312e3f90dfef73ba0288999981694b09affdf6b
> parent 842ac6e867885af041499723dc46f2197705204c
> author Matthieu Moy <Matthieu.Moy@imag.fr> 1441031540 +0200
> committer Matthieu Moy <Matthieu.Moy@imag.fr> 1441031540 +0200
>
> Merge remote-tracking branch 'edward/utf-8-email-support4'
>
> $ git log --no-merges c3c1cc25b27d448e9ef67b265a11be8735ff2df4
> commit c3c1cc25b27d448e9ef67b265a11be8735ff2df4 (grafted)
> Author: Matthieu Moy <Matthieu.Moy@imag.fr>
> Date:   Mon Aug 31 16:32:20 2015 +0200
>
>     Merge remote-tracking branch 'edward/utf-8-email-support4'
>
> I guess Git counts the number of parents that are actually in the
> repository, but it could check the number of "parents" field in the
> object (cat-file -p was still able to show 2).

> cat-file is low-level to show the bare metal, but by using these
> facility you asked Git to give you an imaginary history where that
> commit is the root commit--and that is why it is shown, I think.
>
> What does it do if you say "git -c log.showRoot=false log -p"?

> Junio C Hamano <gitster@pobox.com> writes:
>
>> I do not think this is limited to shallow but for grafts in
>> general.  
>
> Probably yes. I happen to only use grafts in shallow clones ;-).
>
>> cat-file is low-level to show the bare metal, but by using these
>> facility you asked Git to give you an imaginary history where that
>> commit is the root commit--and that is why it is shown, I think.
>>
>> What does it do if you say "git -c log.showRoot=false log -p"?
>
> I get the commit without the patch:
>
> commit c3c1cc25b27d448e9ef67b265a11be8735ff2df4 (grafted)
> Author: Matthieu Moy <Matthieu.Moy@imag.fr>
> Date:   Mon Aug 31 16:32:20 2015 +0200
>
>     Merge remote-tracking branch 'edward/utf-8-email-support4'
>
> Without "-c log.showRoot=false" I get a big patch (diff of the commit
> against the empty tree).