avoid using a git hook

[ktdreyer]

Rather than installing a .git/hooks/prepare-commit-msg file, downstream-cherry-picker should dynamically modify the commit message on the fly during each run.

Some developers have a .git/hooks/prepare-commit-msg file already, and it may have different contents. It would also reduce the things that could go wrong if we cannot install the Gist for some reason, or if we ever have to distribute updates to that Gist.

Alfredo used the following to modify his commit messages to add a "ceph-volume: " prefix:

git filter-branch -f --msg-filter 'sed "s/^/ceph-volume: /"' 6a342e79a1..HEAD

We should do something similar with filter-branch.

[alfredodeza]

For appending at the end I've used:

git filter-branch -f --msg-filter 'cat && echo "\nResolves: rhbz#1485011\n"' master..HEAD