bundle: refuse to create empty bundle

When an user tries to create an empty bundle via `git bundle create
<bundle> <revlist>` where <revlist> resolves to an empty list (for
example, like `master..master`), the command fails and warns the user
about how it don't want to create empty bundle.

However, on Windows the .lock file was still open and could not be
deleted properly. This patch fixes that issue.

This closes #790

Signed-off-by: Gaël Lhez <gael.lhez@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

bundle.c

@@ -454,10 +454,11 @@ int create_bundle(struct bundle_header *header, const char *path,
 	object_array_remove_duplicates(&revs.pending);
 
 	ref_count = write_bundle_refs(bundle_fd, &revs);
-	if (!ref_count)
-		die(_("Refusing to create empty bundle."));
-	else if (ref_count < 0)
+	if (ref_count <= 0)  {
+		if (!ref_count)
+			error(_("Refusing to create empty bundle."));
 		goto err;
+	}
 
 	/* write pack */
 	if (write_pack_data(bundle_fd, &revs)) {

t/t5607-clone-bundle.sh

@@ -71,4 +71,8 @@ test_expect_success 'prerequisites with an empty commit message' '
 	git bundle verify bundle
 '
 
+test_expect_success 'try to create a bundle with empty ref count' '
+	test_expect_code 1 git bundle create foobar.bundle master..master
+'
+
 test_done