pull: don't suggest running hg merge when new heads are on different branches
After a pull when new heads are added but no head is added on the current
branch, the "run 'hg merge'" message can be misleading. This patch doesn't
output the merge message in that scenario.
$ echo '[Section]' >> $HGRCPATH
$ echo 'KeY = Case Sensitive' >> $HGRCPATH
$ echo 'key = lower case' >> $HGRCPATH
$ hg showconfig Section
Section.KeY=Case Sensitive
Section.key=lower case