scmutil.addremove: remove redundant directory and symlink checks
dirstate.walk only does lstats and never returns stat objects for directories.
On a large repository with 170,000 files, this speeds perfaddremove up from
2.40 seconds to 2.34.
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