largefiles: omit restoring standins if working parent is not rollbacked

For efficiency, this patch omits restoring standins and updating
lfdirstate, if the parent of the working directory is not rollbacked.

This patch adds the test not to confirm whether restoring is skipped
or not, but to detect unexpected regression in the future: it is
difficult to distinguish between skipping and perfectly restoring.

hgext/largefiles/overrides.py

@@ -1196,7 +1196,12 b' def overridepurge(orig, ui, repo, *dirs,'
 def overriderollback(orig, ui, repo, **opts):
     wlock = repo.wlock()
     try:
+        before = repo.dirstate.parents()
         result = orig(ui, repo, **opts)
+        after = repo.dirstate.parents()
+        if before == after:
+            return result # no need to restore standins
+
         merge.update(repo, node=None, branchmerge=False, force=True,
                      partial=lfutil.isstandin)
 

tests/test-largefiles-update.t

@@ -129,6 +129,29 b' Test that "hg rollback" restores status '
   $ hg status -A largeY
   ? largeY
 
+Test that "hg rollback" restores standins correctly
+
+  $ hg commit -m 'will be rollback-ed soon'
+  $ hg update -q -C 2
+  $ cat large1
+  large1
+  $ cat .hglf/large1
+  4669e532d5b2c093a78eca010077e708a071bb64
+  $ cat large2
+  large2 in #2
+  $ cat .hglf/large2
+  3cfce6277e7668985707b6887ce56f9f62f6ccd9
+
+  $ hg rollback -q -f
+  $ cat large1
+  large1
+  $ cat .hglf/large1
+  4669e532d5b2c093a78eca010077e708a071bb64
+  $ cat large2
+  large2 in #2
+  $ cat .hglf/large2
+  3cfce6277e7668985707b6887ce56f9f62f6ccd9
+
 Test that "hg status" shows status of largefiles correctly just after
 automated commit like rebase/transplant