gitk: Show branch name(s) as well, if "show nearby tags" is enabled

gitk: Show branch name(s) as well, if "show nearby tags" is enabled

This is a small extension to the code that reads the complete commit
graph, to make it compute descendent heads as well as descendent tags.
We don't exclude descendent heads that are descendents of other
descendent heads as we do for tags, since it is useful to know all the
branches that a commit is on.

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