TO-UNDO: debug

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
git-mktree: fsck the tree object before writing

So far, `git mktree` performs rudimentary checks on the input, but we
should really perform deeper checks lest we introduce invalid tree
objects into the repository.

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

This test verifies that git-mktree refuses to write a tree object that
references non-existing objects.

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