tests: add test coverage for lfdirstate invalidation of linear update

f72d73937853 introduced a significant performance regression: All largefiles
are marked 'normallookup' in lfdirstate by linear (or noop) updates and has to
be rehashed by the next command.

To avoid such regressions, keep an eye on the dirstate content after a plain
'hg up'.

tests/test-largefiles-update.t

@@ -25,6 +25,21 b' directory (and ".hg/largefiles/dirstate"'
   $ hg commit -m '#2'
   created new head
 
+Test that lfdirstate keeps track of last modification of largefiles and
+prevents unnecessary hashing of content - also after linear/noop update
+
+  $ sleep 1
+  $ hg st
+  $ hg debugdirstate --large --nodate
+  n 644          7 large1
+  n 644         13 large2
+  $ hg up
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg debugdirstate --large --nodate
+  n   0         -1 large1
+  n   0         -1 large2
+BAD:            ^^
+
 Test that "hg merge" updates largefiles from "other" correctly
 
 (getting largefiles from "other" normally)