Add a parameter to the backend to allow checking for the old symbolic
target.

Free the old ref even on success.

In case we loose the race to update the reference, return GIT_EMODIFIED
to let the user distinguish it from other types of errors.

Change the name to _matching() intead of _if(), and force _set_target()
to be a conditional update. If the user doesn't care about the old
value, they should use git_reference_create().

Allow updating references if the old value matches the given one.

Corrections for merge working directory validation

Validating the workdir should not compare HEAD to working
directory - this is both inefficient (as it ignores the cache)
and incorrect.  If we had legitimately allowed changes in the
index (identical to the merge result) then comparing HEAD to
workdir would reject these changes as different.  Further, this
will identify files that were filtered strangely as modified,
while testing with the cache would prevent this.

Also, it's stupid slow.

Correct "new" id for reattached-HEAD reflog entry

Add `git_reference_is_note`.

Update to latest clar

Reflog completionism

Index tree-bob collision

If you are checking out NONE, then don't remove.

The checkout code used to defer removal of "blocking" files in
checkouts until the blocked item was actually being written (since
we have already checked that the removing the block is acceptable
according to the update rules).  Unfortunately, this resulted in
an intermediate index state where both the blocking and new items
were in the index which is no longer allowed.  Now we just remove
the blocking item in the first pass so it never needs to coexist.

In cases where there are typechanges, this could result in a bit
more churn of removing and recreating intermediate directories,
but I'm going to assume that is an unusual case and the churn will
not be too costly.