gitk: Fix bug causing the "can't unset idinlist(...)" error

gitk: Fix bug causing the "can't unset idinlist(...)" error

Under some circumstances, having duplicate parents in a commit could
trigger a "can't unset idinlist" Tcl error.  This fixes the cause
(the logic in layoutrows could end up putting the same commit into
rowidlist twice) and also puts a catch around the unset to ignore
the error.

Thanks to Jeff King for coming up with a test script to generate a
repo that shows the problem.

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