gitk: Don't try to list large numbers of tags or heads in the details pane

gitk: Don't try to list large numbers of tags or heads in the details pane

With some large repositories, a commit can end up on thousands of
branches, which results in an extremely long "Branches:" line in the
details window, and that results in the window being extremely slow
to scroll.

This fixes it by just showing "many (N)" after "Branches:", "Follows:"
or "Precedes:", where N is the number of heads or tags.  The limit
is currently set at 20 but could be made configurable (and the "many"
could be a link to pop up a window listing them all in case anyone
really wants to know).

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