Fix and improve the release tool

**tools/release.sh**

 - Fix the command pushing the rel tag.

 - Fix the TAG name. Before this commit, the version number was
   incorrectly surrounded by curly braces, e.g. `v{0.8.0}`.

 - Use the list of changes as a tag message.

**tools/git-changelog**

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.

@3y3 please review.