gitk: Fix file list display when files are renamed

The conversion of the file list to use a text widget assumed incorrectly
that the list of files from git-diff-tree -r would correspond 1-1 with
the diff sections in the output of git-diff-tree -r -p -C, which is
not true when renames are detected.  This fixes it by keeping the
elements in the difffilestart list in the order they appear in the
file list window.

Since this means that the elements of difffilestart are no longer
necessarily in ascending order, it's somewhat hard to do the dynamic
highlighting in the file list as the diff window is scrolled, so I
have taken that out for now.

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