zlib: wrap inflateInit2 used to accept only for gzip format

http-backend.c uses inflateInit2() to tell the library that it wants to
accept only gzip format. Wrap it in a helper function so that readers do
not have to wonder what the magic numbers 15 and 16 are for.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/http-backend.c b/http-backend.c
index c74cb03..ab5015d 100644
--- a/http-backend.c
+++ b/http-backend.c
@@ -275,12 +275,9 @@
 	unsigned char in_buf[8192];
 	unsigned char out_buf[8192];
 	unsigned long cnt = 0;
-	int ret;
 
 	memset(&stream, 0, sizeof(stream));
-	ret = inflateInit2(&stream, (15 + 16));
-	if (ret != Z_OK)
-		die("cannot start zlib inflater, zlib err %d", ret);
+	git_inflate_init_gzip_only(&stream);
 
 	while (1) {
 		ssize_t n = xread(0, in_buf, sizeof(in_buf));