refs/files: handle D/F conflicts during locking

The previous commit added the necessary validation and checks for F/D
conflicts in the files backend when working on case insensitive systems.

There is still a possibility for D/F conflicts. This is a different from
the F/D since for F/D conflicts, there would not be a conflict during
the lock creation phase:

refs/heads/foo.lock
refs/heads/foo/bar.lock

However there would be a conflict when the locks are committed, since we
cannot have 'refs/heads/foo/bar' and 'refs/heads/foo'. These kinds of
conflicts are checked and resolved in
`refs_verify_refnames_available()`, so the previous commit ensured that
for case-insensitive filesystems, we would lowercase the inputs to that
function.

For D/F conflicts, there is a conflict during the lock creation phase
itself:

refs/heads/foo/bar.lock
refs/heads/foo.lock

As in `lock_raw_ref()` after creating the lock, we also check for D/F
conflicts. This can occur in case-insensitive filesystems when trying to
fetch case-conflicted references like:

refs/heads/Foo/new
refs/heads/foo

D/F conflicts can also occur in case-sensitive filesystems, when the
repository already contains a directory with a lock file
'refs/heads/foo/bar.lock' and trying to fetch 'refs/heads/foo'. This
doesn't concern directories containing garbage files as those are
handled on a higher level.

To fix this, simply categorize the error as a name conflict. Also remove
this reference from the list of valid refnames for availability checks.
By categorizing the error and removing it from the list of valid
references, batched updates now knows to reject such reference updates
and apply the other reference updates.

Fix a small typo in `ref_transaction_maybe_set_rejected()` while here.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>