merge: drop reference to file contents after write

This reduces memory usage on large consecutive gets.

mercurial/merge.py

@@ -323,6 +323,7 b' def applyupdates(repo, action, wctx, mct'
             repo.ui.note(_("getting %s\n") % f)
             t = mctx.filectx(f).data()
             repo.wwrite(f, t, flags)
+            t = None
             updated += 1
             if f == '.hgsubstate': # subrepo states need updating
                 subrepo.submerge(repo, wctx, mctx, wctx)