tarballs: git archive: put files in directory

Make the releases created from the "git archive" code path more similar
to the autotools releases, by having all files in a subdirectory with
the project name and tag name instead of having them directly in the
root of the tarball.

  $ tree osmo_dia2gsup-0.1.1 | head
  osmo_dia2gsup-0.1.1
  ├── build_dep.tar.gz
  ├── contrib
  │   ├── generate_build_dep.sh
  │   ├── jenkins.sh
  │   └── systemd
  │       └── osmo_dia2gsup.service
  ├── debian
  │   ├── changelog
  │   ├── compat

Change-Id: If37dcdc04278de0d4479cf72e0ca33c90b7deea8
1 file changed