streaming: void pointer instead of char pointer

Allow any kind of buffer to be fed to read_istream() without an explicit
cast by making it's buf argument a void pointer.  It's about arbitrary
data, not only characters.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

streaming.c

@@ -99,7 +99,7 @@ int close_istream(struct git_istream *st)
 	return r;
 }
 
-ssize_t read_istream(struct git_istream *st, char *buf, size_t sz)
+ssize_t read_istream(struct git_istream *st, void *buf, size_t sz)
 {
 	return st->vtbl->read(st, buf, sz);
 }