bundle: add parse_bundle_header() helper function

Move most of the code from read_bundle_header() to parse_bundle_header()
that takes a file descriptor that is already opened for reading, and make
the former responsible only for opening the file and noticing errors.

As a logical consequence of this, is_bundle() helper function can be
implemented as a non-complaining variant of read_bundle_header() that
does not return an open file descriptor, and can be used to tighten
the check used to decide the use of bundle transport in transport_get()
function.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/transport.c b/transport.c
index c9c8056..84d6480 100644
--- a/transport.c
+++ b/transport.c
@@ -913,7 +913,7 @@
 		ret->fetch = fetch_objs_via_rsync;
 		ret->push = rsync_transport_push;
 		ret->smart_options = NULL;
-	} else if (is_local(url) && is_file(url)) {
+	} else if (is_local(url) && is_file(url) && is_bundle(url, 1)) {
 		struct bundle_transport_data *data = xcalloc(1, sizeof(*data));
 		ret->data = data;
 		ret->get_refs_list = get_refs_from_bundle;