mailinfo: treat header values as C strings

mailinfo: treat header values as C strings

We read each header line into a strbuf, which means that we could
in theory handle header values with embedded NUL bytes. But in practice,
the values we parse out are passed to decode_header(), which uses
strstr(), strchr(), etc. And we would not expect such bytes anyway; they
are forbidden by RFC822, etc. and any non-ASCII characters should be
encoded with RFC2047 encoding.

So let's switch to using strbuf_addstr(), which saves us some length
computations (and will enable further cleanups in this code).

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