gitk: Fix bug causing "can't read commitrow(0,n)" error

gitk: Fix bug causing "can't read commitrow(0,n)" error

In commit 66e46f37de3ed3211a8ae0e8fc09c063bc3a1e08 I changed gitk to
store ids in rowrangelist and idrowranges rather than row numbers,
but I missed two places in the layouttail procedure.  This resulted
in occasional errors such as the "can't read "commitrow(0,8572)":
no such element in array" error reported by Mark Levedahl.  This fixes
it by using the id rather than the row number.

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