Wrap inflate and other zlib routines for better error reporting

R. Tyler Ballance reported a mysterious transient repository corruption;
after much digging, it turns out that we were not catching and reporting
memory allocation errors from some calls we make to zlib.

This one _just_ wraps things; it doesn't do the "retry on low memory
error" part, at least not yet. It is an independent issue from the
reporting.  Some of the errors are expected and passed back to the caller,
but we die when zlib reports it failed to allocate memory for now.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-unpack-objects.c

@@ -99,10 +99,10 @@ static void *get_data(unsigned long size)
 	stream.avail_out = size;
 	stream.next_in = fill(1);
 	stream.avail_in = len;
-	inflateInit(&stream);
+	git_inflate_init(&stream);
 
 	for (;;) {
-		int ret = inflate(&stream, 0);
+		int ret = git_inflate(&stream, 0);
 		use(len - stream.avail_in);
 		if (stream.total_out == size && ret == Z_STREAM_END)
 			break;
@@ -118,7 +118,7 @@ static void *get_data(unsigned long size)
 		stream.next_in = fill(1);
 		stream.avail_in = len;
 	}
-	inflateEnd(&stream);
+	git_inflate_end(&stream);
 	return buf;
 }