dirstate: only call lstat once per flags invocation
This makes a big difference to performance in some cases.
hg --time locate 'set:symlink()'
mozilla-central (70,000 files):
before: 2.92 sec
after: 2.47
another repo (170,000 files):
before: 7.87 sec
after: 6.86
http://mercurial.selenic.com/bts/issue1438
$ "$TESTDIR/hghave" symlink || exit 80
$ hg init
$ ln -s foo link
$ hg add link
$ hg ci -mbad link
$ hg rm link
$ hg ci -mok
$ hg diff -g -r 0:1 > bad.patch
$ hg up 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg import --no-commit bad.patch
applying bad.patch
$ hg status
R link
? bad.patch