gitk: Fix the find and highlight functions

gitk: Fix the find and highlight functions

This reworks the way that the "Find" button (and the /, ?, ^F, and ^G
keys) works.  Previously, pressing the "Find" button would cause gitk
to go off and scan through every commit to see which commits matched,
and the user interface was completely unreponsive during that time.
Now the searching is done in chunks using the scheduler, so the UI
still responds, and the search stops as soon as a matching commit is
found.

The highlighting of matches using a yellow background is now done in
the commit-drawing code and the highlighting code.  This ensures that
all the commits that are visible that match are highlighted without
the search code having to find them all.

This also fixes a bug where previously-drawn commits that need to be
highlighted were not being highlighted.

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