large-files: make sure we write newly initialized standin file early

Any changing context will have to initialized it before anything else. Not
flushing the default (pre-change) content mean we would enter the changing
context with a dirty dirstate, which is odd.

hgext/largefiles/lfutil.py

@@ -249,6 +249,8 b' def openlfdirstate(ui, repo, create=True'
                         wc_tracked=True,
                         possibly_dirty=True,
                     )
+                # avoid getting dirty dirstate before other operations
+                lfdirstate.write(repo.currenttransaction())
         except error.LockError:
             # Assume that whatever was holding the lock was important.
             # If we were doing something important, we would already have