The .gitattributes cache should not reload .gitattributes in the
middle of checking out, only between checkout operations.  Otherwise,
we'll spend all our time stat'ing and read'ing the gitattributes.

Checkout can now provide performance data about the number of (some)
syscalls performed using an optional callback.

On a case-insensitive filesystem, we need to deal with case-changing
renames (eg, foo -> FOO) by removing the old and adding the new,
exactly as if we were on a case-sensitive filesystem.

Update the `checkout::tree::can_cancel_checkout_from_notify` test, now
that notifications are always sent case sensitively.

Valgrind is now clean except for libssl and libgcrypt.

Signed-off-by: Sven Strickroth <email@cs-ware.de>

This was not converted when we converted the rest, so do it now.

Don't try to determine whether the system supports file modes
when putting the tree data in the index during checkout.  The tree's
mode is canonical and did not come from stat(2) in the first place.

Any well-behaved program should write a descriptive message to the
reflog whenever it updates a reference. Let's make this more prominent
by removing the version without the reflog parameters.

The checkout notify callback behavior on non-zero return values
was not being tested.  This adds tests, fixes a bug with positive
values, and clarifies the documentation to make it clear that the
checkout can be canceled via this mechanism.

Prevent checkout tree when unresolved changes exist (unless FORCE flag
is specified).  Clear NAME table when checking out, to avoid
checkout_conflicts from attempting to manipulate it.  Ensure that NAME
is also cleared at reset.

This adds additonal tests of the checkout target directory option
including using it to dump data from bare repos.

This fixes the checkout case when a file is modified between the
baseline and the target and yet missing in the working directory.
The logic for that case appears to have been wrong.

This also adds a useful checkout notify callback to the checkout
test helpers that will count notifications and also has a debug
mode to visualize what checkout thinks that it's doing.

This fixes of the file contents checks in checkout to give
slightly better error messages by directly calling the underlying
clar assertions so the file and line number of the top level call
can be reported correctly, and renames the helpers to not start
with "test_" since that is kind of reserved by clar.

This also enables some of the CRLF tests on all platforms that
were previously Windows only (by pushing a check of the native
line endings into the test body).

This fixes some places where the new tests were leaving the test
area in a bad state or were freeing data they should not free.
It also removes code that is extraneous to the core issue and
fixes an invalid SHA being looked up in one of the tests (which
was failing, but for the wrong reason).

Signed-off-by: Sven Strickroth <email@cs-ware.de>

This adds a helper function for the cases where you want to
quickly set a single boolean config value for a repository.
This allowed me to remove a lot of code.