script/ceph-backport.sh: Add prepare function

script/ceph-backport.sh: Add prepare function

Add a new flag, --prepare, that when used will fetch all necessary info and
cherrypick all original commits automatically.
It will also prevent the main code to be executed.

After running with --prepare, and if there were no conflicts, you can simply
remove the flag from the command and run the script again.
This time it will run the main code and create the PR and update the backport
issue.

p.e.:
ceph-backport.sh 19206 jewel --prepare
ceph-backport.sh 19206 jewel

Signed-off-by: Tiago Melo <tmelo@suse.com>