log: make sure the passed rev exists or bailed out early
bug-log: rewrite in order to support --cherry-* options

Those options can be used to inspect the bug differences between 2
branches. It can shows which new bugs have been fixed or which ones
have been lost.

--cherry-mark is the same as --cherry-pick but also shows the common
fixes in both branches.

This mimic the behaviour of "git log --cherry-*".
log: use new --cherry-pick option with kdist-bug-log
bug-fix: hint the user for pushing bug annotations
bug-import: fix typo in stat_updated
bug-import: hint the user for pushing bug annotations
bug-import: adjust spaces when reporting
mdv: use a new scheme for rpm kernel package post installation

Simplify the rpm scriptlet for kernel package: we don't rely in
bootloader-utils (and drakx) to generate a new initrd and to update
bootloader menu entry.

Instead a simple scheme is used: tools that want to be notified when
installing a new kernel should put an executable script in
/etc/kernel/*.d directories. Those scripts will be executed whenever a
kernel binary package is installed or removed.

Also this part was previously x86 specific, but it's now part of the
common arch stuff because tools used by scriptlets are now optional.