gitk: Avoid a crash in selectline if commitinfo($id) isn't set
gitk: Avoid a crash in selectline if commitinfo($id) isn't set

Occasionally I see a crash in selectline with commitinfo($id) not
set.  This makes sure it is set by calling getcommit $id if it isn't.

Signed-off-by: Paul Mackerras <19a0ba370c443ba08d20b5061586430ab449ee8c@samba.org>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git