filemerge: simplify slightly by using filectx.decodeddata()

Differential Revision: https://phab.mercurial-scm.org/D11802

mercurial/filemerge.py

@@ -995,7 +995,7 b' def _maketempfiles(repo, fco, fca, local'
 
     def tempfromcontext(prefix, ctx):
         f, name = maketempfrompath(prefix, ctx.path())
-        data = repo.wwritedata(ctx.path(), ctx.data())
+        data = ctx.decodeddata()
         f.write(data)
         f.close()
         return name