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>

cache.h

@@ -21,6 +21,7 @@
 #endif
 
 void git_inflate_init(z_streamp strm);
+void git_inflate_init_gzip_only(z_streamp strm);
 void git_inflate_end(z_streamp strm);
 int git_inflate(z_streamp strm, int flush);