gitk: Store ids in rowrangelist and idrowranges rather than row numbers

gitk: Store ids in rowrangelist and idrowranges rather than row numbers

This removes the need for insertrow to go through rowrangelist and
idrowranges and adjust a lot of entries.  The first entry for a given
id is now the row number of the first child, not that row number + 1,
and rowranges compensates for that so its callers didn't have to
change.  This adds a ranges argument to drawlineseg so that we can
avoid calling rowranges a second time inside drawlineseg (all its
callers already called rowranges).

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