D6169: unshelve: disable unshelve during merge (issue5123)

[Yuya Nishihara]

>   > Is it difficult to fix unshelve to not lose merge parents?
>   > 
>   > https://bz.mercurial-scm.org/show_bug.cgi?id=5123#c2
>   
>   I'll try to come up with a fix. But, `hg shelve` is aborting during a merge. Can `unshelve` be aborted too?

It could abort, but mpm said there would be no reason to, and I agree with
that. Unshelving on top of a merge seems somewhat useful.