--quiet should suppress stderr output

[rraval]

Tab completion scripts may wish to execute git-nomad in non-.git directories and it's annoying to have to redirect stderr.

$ git-nomad ls --print=ref --quiet                                                                                                                                                                                         
Error: command failure
$ cd "/home/rraval" && "git" "rev-parse" "--absolute-git-dir"
# status: exit status: 128
# ---- STDERR ----
fatal: not a git repository (or any of the parent directories): .git

This should output nothing and just set a non-zero exit status.