D6169: unshelve: disable unshelve during merge (issue5123)

[Yuya Nishihara]

>   I saw some patches[0][1][2] trying to fix this issue. It seems like it's difficult to preserve the second parent.
>   
>   [0]: http://mercurial.808500.n3.nabble.com/PATCH-shelve-adds-restoring-original-parents-after-unshelve-issue5123-tc4035657.html#none
>   [1]: http://mercurial.808500.n3.nabble.com/PATCH-V2-shelve-restore-parents-after-unshelve-issue5123-tc4036093.html#a4037370
>   [2]: http://mercurial.808500.n3.nabble.com/PATCH-V3-shelve-restore-parents-after-unshelve-issue5123-tt4036858.html#a4037408

Okay, let's disable unshelve then. Can you update the commit message to
include why we can't simply allow it?