strbuf: add strbuf_tolower function

strbuf: add strbuf_tolower function

This is a convenience wrapper to call tolower on each
character of the string.

This makes config's lowercase() function obsolete, though
note that because we have a strbuf, we are careful to
operate over the whole strbuf, rather than assuming that a
NUL is the end-of-string.

We could continue to offer a pure-string lowercase, but
there would be no callers (in most pure-string cases, we
actually duplicate and lowercase the duplicate, for which we
have the xstrdup_tolower wrapper).

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