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 base
abort: cannot delete applied patch c
Now at: b
a
b
a
b
series
status
Now at: a
a
b
series
status
1 New patch: a
0 base
abort: cannot delete revision 3 above applied patches
f
4 New patch: f
3 New patch: e
2 New patch: d
1 New patch: a
0 base