tags: visit new heads in forward order when rebuilding cache
This improves performance of building the tag cache by 4x on a repo
with ~2800 heads.
hide outer repo
$ hg init
$ echo '[Section]' >> $HGRCPATH
$ echo 'KeY = Case Sensitive' >> $HGRCPATH
$ echo 'key = lower case' >> $HGRCPATH
$ hg showconfig Section
Section.KeY=Case Sensitive
Section.key=lower case