WIP: git-mktree: fsck the tree object before writing

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
WIP: Add tests for mktree validation

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
TO-UNDO: debug

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>