- when user provided a $SOURCE argument, do call abs_path on
  it. abs_path allows the user to provide relative paths in
  the original directory.

- don't call abs_path on "$GITDIR/NetworkManager-$VERSION".tar.xz
  abs_path is there to verify user input and it will abort the script
  if the file doesn't exist.

- when creating a temporary tarball via git-archive, put it
  into the output directory and not overwriting files in
  $GITDIR.

- fix abs_path to return an error code and let callers abort
  the script

- add a new parameter $SOURCE_FROM_GIT so that you can control
  whether git-archive is used. You can now specify either $SOURCE
  or $SOURCE_FROM_GIT. In case of absence of both, it tries to
  detect a tar file in the $GITDIR directory.

Fixes: 9e9ec1a3