gitk: Further speedups

Now we don't parse the commits as we are reading them, we just put
commit data on a list as a blob, and instead parse the commit when
we need the various parts of it, such as when a commit is drawn on
the canvas.  This makes searching a bit more interesting: now we
scan through the commit blobs doing a string or regexp match to find
commits that might match, then for those that might match, we parse
the commit info (if it isn't already parsed) and do the matching
for the various fields as before.

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