zlib: wrap inflateInit2 used to accept only for gzip format

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>