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 foo
adding bomb
adding a.c
adding dir/a.o
adding dir/b.o
! a.c
! dir/a.o
! dir/b.o
? .hgignore
a.c: unsupported file type (type is fifo)
! a.c
# test absolute path through symlink outside repo
A f
# try symlink outside repo to file inside
abort: ../z not under root