build: Remove more keywords from the generated ChangeLog

* build-aux/gitlog-to-changelog (parse_amend_file): Generalize keyword
removal.

This simplified rule does now match most special keywords like

Signed-off-by:
GnuPG-bug-id:
Fixes-commit:

However it does not match the sometimes used

Fixes:

which should be avoided anyway.

This patch helps in cases where the "--" delimiter line was not used
and only keywords were given.

• Signed-off-by: Werner Koch <wk@gnupg.org>