commitpkg: Fix leading whitespace in commit messages

Commit c51cc8d3

 erroneously introduced a
tab character that is prepended to the body of each commit message. Fix
this by using C-style newline escape sequences which will avoid further
whitespace issues caused by re-indentation of code at this point.

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