extdiff: open files in binary mode

Problem noticed by Darrell Gallion.

hgext/extdiff.py

@@ -77,7 +77,7 b' def dodiff(ui, repo, diffcmd, diffopts, '
             destdir = os.path.dirname(dest)
             if not os.path.isdir(destdir):
                 os.makedirs(destdir)
-            repo.wwrite(wfn, repo.file(fn).read(mf[fn]), open(dest, 'w'))
+            repo.wwrite(wfn, repo.file(fn).read(mf[fn]), open(dest, 'wb'))
         return dirname
 
     def snapshot_wdir(files):
@@ -99,7 +99,7 b' def dodiff(ui, repo, diffcmd, diffopts, '
             destdir = os.path.dirname(dest)
             if not os.path.isdir(destdir):
                 os.makedirs(destdir)
-            fp = open(dest, 'w')
+            fp = open(dest, 'wb')
             for chunk in util.filechunkiter(repo.wopener(wfn)):
                 fp.write(chunk)
         return dirname