t0021: tr portability fix for Solaris

Solaris' /usr/bin/tr doesn't seem to like multiple character
ranges in brackets (it simply prints "Bad string").

Instead, let's just enumerate the transformation we want.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0021-conversion.sh

@@ -5,7 +5,9 @@ test_description='blob conversion via gitattributes'
 . ./test-lib.sh
 
 cat <<\EOF >rot13.sh
-tr '[a-zA-Z]' '[n-za-mN-ZA-M]'
+tr \
+  'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ' \
+  'nopqrstuvwxyzabcdefghijklmNOPQRSTUVWXYZABCDEFGHIJKLM'
 EOF
 chmod +x rot13.sh