refs.c: pass a skip list to name_conflict_fn

Allow passing a list of refs to skip checking to name_conflict_fn.
There are some conditions where we want to allow a temporary conflict and skip
checking those refs. For example if we have a transaction that
1, guarantees that m is a packed refs and there is no loose ref for m
2, the transaction will delete m from the packed ref
3, the transaction will create conflicting m/m

For this case we want to be able to lock and create m/m since we know that the
conflict is only transient. I.e. the conflict will be automatically resolved
by the transaction when it deletes m.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: propagate any errno==ENOTDIR from _commit back to the callers

In _commit, ENOTDIR can happen in the call to lock_ref_sha1_basic, either when
we lstat the new refname and it returns ENOTDIR or if the name checking
function reports that the same type of conflict happened. In both cases it
means that we can not create the new ref due to a name conflict.

For these cases, save the errno value and abort and make sure that the caller
can see errno==ENOTDIR.

Also start defining specific return codes for _commit, assign -1 as a generic
error and -2 as the error that refers to a name conflict. Callers can (and
should) use that return value inspecting errno directly.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: make rename_ref use a transaction

Change rename_ref to use a single transaction to perform the ref rename.
Add a special flag for _delete so that we know this is a rename so we can
do magic delete and unlinks for the old ref before we start writing out the
new ref.
We have to do these deletes and creates in a careful order in order to
handle n -> n/n and n/n -> n renames since n will change between being a
directory and a file and this restricts when we can allow n/n.lock to exist.

Update test t5505. Since we now try to remove the parent directory for the old
ref during transaction commit for the rename (so that rename n/n -> n still
works) we do not need to rmdir it from the test.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
fetch.c: change s_update_ref to use a ref transaction

Change s_update_ref to use a ref transaction for the ref update.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: make write_ref_sha1 static

No external users call write_ref_sha1 any more so lets declare it static.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>