shelve: don't swallow errors that happen when reverting the working copy

Same reasoning as the parent commit. The code in question here comes
from fb8065de47b0 (unshelve: silence internal revert, 2014-08-15),
which says that it was to prepare for some upcoming revert changes. I
suppose those changes have happened by now but we fixed something
about it, because I don't see any unwanted changes in tests (no
changes at all in tests, in fact).

Differential Revision: https://phab.mercurial-scm.org/D10883

mercurial/shelve.py

@@ -781,9 +781,7 b' def mergefiles(ui, repo, wctx, shelvectx'
     dirstate."""
     with ui.configoverride({(b'ui', b'quiet'): True}):
         hg.update(repo, wctx.node())
-        ui.pushbuffer(True)
         cmdutil.revert(ui, repo, shelvectx)
-        ui.popbuffer()
 
 
 def restorebranch(ui, repo, branchtorestore):