mailinfo: use strcmp() for string comparison

mailinfo: use strcmp() for string comparison

The array header is defined as:

	static const char *header[MAX_HDR_PARSED] = {
	     "From","Subject","Date",
	};

When looking for the index of a specfic string in that array, simply
use strcmp() instead of memcmp().  This avoids running over the end of
the string (e.g. with memcmp("Subject", "From", 7)) and gets rid of
magic string length constants.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>