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>