commitpkg: Provide a commit message template

Generate a message template when omitting the commit message (includes
the same subject line that is used if a commit message is specified).

Also, use two newlines to separate the subject line from the message
body as it looks much clearer that way.

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>