gitk: Clear array 'commitinfo' on reload

After a reload we might have an entirely different set of commits, so keeping all of them leaks memory. Remove them all because re-creating them is not more expensive than testing wether they're still valid. Lazy (re-)creation is already well established, so a missing entry can't cause harm. Signed-off-by: Markus Hitter <mah@jump-ing.de> Signed-off-by: Paul Mackerras <paulus@ozlabs.org>