Tagnames in configuration can include spaces (and other nasties) e.g. in
repository-specific configuration options due to Origin/Label
potentially containing a space. The configuration file format supports
parsing quoted as well as encoded spaces, but the output generated by
apt-config and other places which might be feedback into apt via
parsing (e.g. before calling apt-key in our gpgv method) do not quote
and hence produce invalid configuration files.
Changing the default to be an encoded tagname ensures that the output of
dump can be used as a config file, but other users might not expect
this so that is technically a backward-breaking change.