Store empty (default) branch in branch cache, too. Operations on this branch don't work otherwise. Reading branches.cache had to be adjusted to allow an empty label. Adjusted reading of the cache tip for symmetry, no functional change here.

adding a
diff -r 107ba6f817b5 -r 310ce7989cdc a
--- a/a	Thu Jan 01 00:00:01 1970 +0000
+++ b/a	Thu Jan 01 00:00:02 1970 +0000
@@ -1,2 +1,3 @@ confuse str.splitlines
 confuse str.splitlines
 embedded
newline
+clean diff