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.
diff -r 4da5fa99f904 b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123
diff -r 4da5fa99f904 b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123
diff -r 4da5fa99f904 a
--- a/a
+++ b/a
@@ -0,0 +1,1 @@
+foo
diff -r 4da5fa99f904 b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123
abort: Ambiguous identifier!
abort: Ambiguous identifier!