This started as a complex new test for checkout going through the
"typechanges" test repository, but that revealed numerous issues
with checkout, including:
* complete failure with submodules
* failure to create blobs with exec bits
* problems when replacing a tree with a blob because the tree
"example/" sorts after the blob "example" so the delete was
being processed after the single file blob was created
This fixes most of those problems and includes a number of other
minor changes that made it easier to do that, including improving
the TYPECHANGE support in diff/status, etc.
This is a big redesign of the git_submodule_status API and the
implementation of the redesigned API. It also fixes a number of
bugs that I found in other parts of the submodule API while
writing the tests for the status part.
This also fixes a couple of bugs in the iterators that had not
been noticed before - one with iterating when there is a gitlink
(i.e. separate-work-dir) and one where I was treating anything
even vaguely submodule-like as a submodule, more aggressively
than core git does.
