gitk: Don't compare fake children when comparing commits

gitk: Don't compare fake children when comparing commits

This fixes a bug where the compare-commits function would advance
to a fake node (one representing local changes, either checked in
but not committed, or not checked in) and then get an error when
trying to get the patch-id.  This fixes it by only considering the
real children of each commit.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>