gitk: Various speed improvements

This rearranges the code a little to eliminate some procedure calls
and reduce the number of globals accessed.  It makes rowidlist and
rowoffsets lists rather than arrays, and removes the lineid array,
since $lineid($l) was the same as [lindex $displayorder $l], and the
latter is a little faster.
Signed-off-by: NPaul Mackerras <paulus@samba.org>