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.
Renamed git_checkout_index to what it really was,
and removed duplicate code from clone.c. Added
git_checkout_ref, which updates HEAD and hands off
to git_checkout_head.
Added tests for the options the caller can pass to
git_checkout_*.