url: fix UnicodeDecodeError on certificate verification error
SSLSocket.getpeercert() returns tuple containing unicode for 'subject'.
Since Mercurial does't support IDN at all, it just returns error for non-ascii
certname.
$ hg init
$ python -c 'print "confuse str.splitlines\nembedded\rnewline"' > a
$ hg ci -Ama -d '1 0'
adding a
$ echo clean diff >> a
$ hg ci -mb -d '2 0'
$ hg diff -r0 -r1
diff -r 107ba6f817b5 -r 310ce7989cdc a
--- a/a Thu Jan 01 00:00:01 1970 +0000
+++ b/a Thu Jan 01 00:00:02 1970 +0000
@@ -1,2 +1,3 @@
confuse str.splitlines
embedded\rnewline (esc)
+clean diff