This changes the style of push events that remove tags or branches so
they don't display the commit details. This prevents displaying commit
details such as:

    000000 . --broken encoding

Instead we now simply display the header such as:

    Administrator deleted branch example-branch

This is displayed in the same style as events for newly created
branches/tags.

This commit also ensures that if no commit message is present we simply
don't display anything, instead of "--broken encoding".

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/36685
Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/36722