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.
% new file
applying patch from stdin
% new empty file
applying patch from stdin
empty
% chmod +x
applying patch from stdin
% copy
applying patch from stdin
a
a
% rename
applying patch from stdin
copyx
empty
new
rename
% delete
applying patch from stdin
empty
new
rename
% regular diff
applying patch from stdin
% copy and modify
applying patch from stdin
a
a
b
a
a
% rename and modify
applying patch from stdin
copy2: No such file or directory
a
a
b
c
a
% one file renamed multiple times
applying patch from stdin
9 rename2 rename3 rename3-2 / rename3 (rename2)rename3-2 (rename2)
rename2: No such file or directory
rename3
rename3-2
a
a
b
c
a
a
a
b
c
a
% binary files and regular patch hunks
applying patch from stdin
foo
045c85ba38952325e126c70962cc0f9d9077bc67 644 binary
% many binary files
applying patch from stdin
045c85ba38952325e126c70962cc0f9d9077bc67 644 mbinary1
a874b471193996e7cb034bb301cac7bdaf3e3f46 644 mbinary2