This adds a context-menu command to put a mark on this commit. There
is at most one marked commit at any time, and it is indicated by a box
drawn around the headline. Once a commit is marked, two other
context-menu commands become available: one to select the marked commit,
and another to find the closest common descendant of this commit and
the marked commit.
The "find common descendant" command uses the displayed parent/child
relationships (i.e. the rewritten parent pointers produced by git log),
not the real parent/child relationships. Currently the UI will be
unresponsive while gitk is working out the nearest common descendant;
this should be improved in future.