gitk: Display important heads even when there are many

gitk: Display important heads even when there are many

When there are more than $maxrefs descendant heads to display in the
Branches field of the commit display, we currently just display "many",
which is not very informative.  To make the display more informative,
we now look for "master" and whichever head is currently checked out,
and display them even if there are too many heads to display them all.
The display then looks like "Branches: master and many more (33)" for
instance.

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