The function `refdb_reflog_fs__delete` uses the `if (!error && foobar())`
pattern of checking, where error conditions are being checked by following calls
to different code. This does not match our current style, where the call-site of
a function is usually directly responsible for checking the return value.

Convert the function to use `if ((error = foobar()) < 0) goto out;` style. Note
that this changes the code flow a bit: previously, we were always trying to
delete empty reference hierarchies even if deleting the reflog entry has failed.
This wasn't much of a problem -- if deletion failed, the hierarchy will still
contain at least one file and thus the function call was an expensive no-op.
Now, we will only perform this deletion if we have successfully removed the
reflog.