perf: close transaction in perffncachewrite

This fixes a bug, and brings CPython behaviour on this test into
line with PyPy.

contrib/perf.py

@@ -441,6 +441,7 @@ def perffncachewrite(ui, repo, **opts):
         s.fncache.write(tr)
     timer(d)
     lock.release()
+    tr.close()
     fm.end()
 
 @command('perffncacheencode', formatteropts)

tests/test-contrib-perf.t

@@ -123,8 +123,6 @@ perfstatus
   $ hg perffncacheencode
   $ hg perffncacheload
   $ hg perffncachewrite
-  transaction abort!
-  rollback completed
   $ hg perfheads
   $ hg perfindex
   $ hg perfloadmarkers