buildpackage: Make debian-tag message configurable

New config option --git-debian-tag-msg allows for the specification of
the message format string for signed debian-tags.

When left unset, the default debian-tag-msg format is still:

    %(pkg)s Debian release %(version)s

Signed-off-by: Kamal Mostafa <kamal@whence.com>
Improve error messages on formatting errors

Make it easier for the user to detect misformated replacement strings in
config files and command line options.
gbp: add --version option

Closes: #758909
Allow to list all available commands
Unify doc strings a bit

since they now show up with --list-cmds
bash completion: make command list dynamic

Use "gbp --list-cmds" so we don't have to hardcode the available
commands and get support for the RPM ones as they show up.