gitk: Add a menu item for cherry-picking commits

This does a git-cherry-pick -r to cherry-pick the commit that was
right-clicked on to the head of the current branch.  This would work
better with some minor changes to the git-cherry-pick script.

Along the way, this changes desc_heads to record the names of the
descendent heads rather than their IDs.

Signed-off-by: Paul Mackerras <paulus@samba.org>