cache-tree: avoid an unnecessary check
The first thing the `parse_tree()` function does is to return early if
the tree has already been parsed. Therefore we do not need to guard the
`parse_tree()` call behind a check of that flag.
As of time of writing, there are no other instances of this in Git's
code bases: whenever the `parsed` flag guards a `parse_tree()` call, it
guards more than just that call.
Suggested-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>