Mark files in t/t5100 as UTF-8

This enables gitk to show the patch text with correct glyphs if the locale
is not UTF-8.
Signed-off-by: NJohannes Sixt <j6t@kdbg.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t5100/.gitattributes

+msg*	encoding=UTF-8
+info*	encoding=UTF-8
+rfc2047-info-*	encoding=UTF-8
+sample.mbox	encoding=UTF-8