tools/release: fixes and improvements

 - Fix the command pushing the rel tag.

 - Fix the TAG name. Before this commit, the version number was
   surrounded by curly braces, e.g. `{0.8.0}`.

 - Use the list of changes as a tag message.
tools/git-changelog: always print the header

Modify the changelog generator to always print the header, even when
running with `-l` parameter.

This way the output of `git-changelog -l` makes a proper git commit
message with summary (the header) and a body.