gitk: Fix bug in fix for warning when removing a branch

My fix in commit b1054ac985aebc90c0a78202dab8477b74d7818a was only
half-right, since it ignored the case where the descendent heads of
the head being removed correspond to two or more different commits.
This fixes it.  Reported by Mark Levedahl.

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