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.

     1

%% Should show unknown

     2

? unknown

     3

removing b

     4

%% Should show unknown and b removed

     5

R b

     6

? unknown

     7

%% Should show a and unknown

     8

a

     9

unknown