largefiles: restore standins from non branch-tip parent at rollback correctly

Before this patch, "hg rollback" can't restore standins correclty, if:

  - old parent of the working directory is rollback-ed, and
  - new parent of the working directory is not branch-tip

"overriderollback" uses "merge.update" as a kind of "revert" utility
to restore only standins with "node=None", and this makes
"merge.update" choose "branch-tip" revision as the updating target
unexpectedly.

Then, "merge.update" restores standins from the branch-tip revision
regardless of the parent of the working directory after rollback and
this may cause unexpected behavior.

This patch invokes "merge.update" with "node='.'" to restore standins
from the parent revision of the working directory.

In fact, this "merge.update" invocation will be replaced in the
subsequent patch to fix another problem, but this change is usefull to
inform reason why such complicated case should be tested.

hgext/largefiles/overrides.py

@@ -1202,7 +1202,7 b' def overriderollback(orig, ui, repo, **o'
         if before == after:
             return result # no need to restore standins
 
-        merge.update(repo, node=None, branchmerge=False, force=True,
+        merge.update(repo, node='.', branchmerge=False, force=True,
                      partial=lfutil.isstandin)
 
         lfdirstate = lfutil.openlfdirstate(ui, repo)

tests/test-largefiles-update.t

@@ -152,6 +152,25 b' Test that "hg rollback" restores standin'
   $ cat .hglf/large2
   3cfce6277e7668985707b6887ce56f9f62f6ccd9
 
+(rollback the parent of the working directory, when the parent of it
+is not branch-tip)
+
+  $ hg update -q -C 1
+  $ cat .hglf/large1
+  58e24f733a964da346e2407a2bee99d9001184f5
+  $ cat .hglf/large2
+  1deebade43c8c498a3c8daddac0244dc55d1331d
+
+  $ echo normalX > normalX
+  $ hg add normalX
+  $ hg commit -m 'will be rollback-ed soon'
+  $ hg rollback -q
+
+  $ cat .hglf/large1
+  58e24f733a964da346e2407a2bee99d9001184f5
+  $ cat .hglf/large2
+  1deebade43c8c498a3c8daddac0244dc55d1331d
+
 Test that "hg status" shows status of largefiles correctly just after
 automated commit like rebase/transplant