Make it possible to cherry-pick a change

Option -d already makes it possible to get a change into a separate
branch, but for large projects and trivial changes, it makes sense to
also be able to just cherry-pick the change to the currently active
local branch.

Also add modes that invokes "git cherry-pick -n" or "git cherry-pick
-x".  Program exits with exit code 69 whenever "git cherry-pick" fails.

This feature was subsequently amended by Marcin Cieślak
<saper@saper.info>.

Change-Id: I0a30052fb2baf357ed81e3f1ef4618bf786cb7ee