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>
fetch.c: use a single ref transaction for all ref updates

Change store_updated_refs to use a single ref transaction for all refs that
are updated during the fetch. This makes the fetch more atomic when update
failures occur.

Since ref update failures will now no longer occur in the code path for
updating a single ref in s_update_ref, we no longer have as detailed error
message logging the exact reference and the ref log action as in the old cod
Instead since we fail the entire transaction we log a much more generic
message. But since we commit the transaction using MSG_ON_ERR we will log
an error containing the ref name if either locking of writing the ref would
so the regression in the log message is minor.

This will also change the order in which errors are checked for and logged
which may alter which error will be logged if there are multiple errors
occuring during a fetch.

For example, assume we have a fetch for two refs that both would fail.
Where the first ref would fail with ENOTDIR due to a directory in the ref
path not existing, and the second ref in the fetch would fail due to
the check in update_logical_ref():
        if (current_branch &&
            !strcmp(ref->name, current_branch->name) &&
            !(update_head_ok || is_bare_repository()) &&
            !is_null_sha1(ref->old_sha1)) {
                /*
                 * If this is the head, and it's not okay to update
                 * the head, and the old value of the head isn't empty...
                 */

In the old code since we would update the refs one ref at a time we would
first fail the ENOTDIR and then fail the second update of HEAD as well.
But since the first ref failed with ENOTDIR we would eventually fail the who
fetch with STORE_REF_ERROR_DF_CONFLICT

In the new code, since we defer committing the transaction until all refs
have been processed, we would now detect that the second ref was bad and
rollback the transaction before we would even try start writing the update t
disk and thus we would not return STORE_REF_ERROR_DF_CONFLICT for this case.

I think this new behaviour is more correct, since if there was a problem
we would not even try to commit the transaction but need to highlight this
change in how/what errors are reported.
This change in what error is returned only occurs if there are multiple
refs that fail to update and only some, but not all, of them fail due to
ENOTDIR.

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