Mercurial: make sure `shelve` uses the pager

Useful when doing `hg shelve --list --patch`, especially seeing as
Mercurial doesn't provide any way of showing the patch for just one of
the shelved changes (you have to show the patch for all of them, which
is more likely to be more than a screenful).

Signed-off-by: Greg Hurrell <greg@hurrell.net>