update: don't translate the abort message twice

The string representation of util.Abort() is translated when
merge.update() raises the exception.  For languages with characters
out of the valid ascii range, if we feed them again to i18n.gettext()
mercurial dies with:

[...]
  File "/home/javi/src/mercurial/mercurial/hg-mpm/mercurial/commands.py", line 4287, in postincoming
    ui.warn(_("not updating: %s\n" % str(inst)))
  File "/home/javi/src/mercurial/mercurial/hg-mpm/mercurial/i18n.py", line 42, in gettext
    u = u'\n\n'.join([p and t.ugettext(p) or '' for p in paragraphs])
  File "/usr/lib/python2.7/gettext.py", line 404, in ugettext
    return unicode(message)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xd0 in position 14: ordinal not in range(128)

To reproduce this error, just try to pull a changeset that crosses
branches with LANG=ru_RU.UTF-8

mercurial/commands.py

@@ -4269,7 +4269,7 b' def postincoming(ui, repo, modheads, opt'
         try:
             ret = hg.update(repo, checkout)
         except util.Abort, inst:
-            ui.warn(_("not updating: %s\n" % str(inst)))
+            ui.warn(_("not updating: %s\n") % str(inst))
             return 0
         if not ret and not checkout:
             if bookmarks.update(repo, [movemarkfrom], repo['.'].node()):