gitk: Show nearby tags

gitk: Show nearby tags

This adds a feature to the diff display window where it will show
the tags that this commit follows (is a descendent of) and precedes
(is an ancestor of).  Specifically, it will show the tags for all
tagged descendents that are not a descendent of another tagged
descendent of this commit, and the tags for all tagged ancestors
that are not ancestors of another tagged ancestor of this commit.

To do this, gitk reads the complete commit graph using git rev-list
and performs a couple of traversals of the tree.  This is done in
the background, but since it can be time-consuming, there is an option
to turn it off in the `edit preferences' window.

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