Re: [PATCH 04/17] add_to_alternates_file: don't add duplicate entries

> The add_to_alternates_file function blindly uses
> hold_lock_file_for_append to copy the existing contents, and
> then adds the new line to it. This has two minor problems:
> 
>   1. We might add duplicate entries, which are ugly and
>      inefficient.
> 
>   2. We do not check that the file ends with a newline, in
>      which case we would bogusly append to the final line.
>      This is quite unlikely in practice, though, as we call
>      this function only from git-clone, so presumably we are
>      the only writers of the file (and we always add a
>      newline).
> 
> Instead of using hold_lock_file_for_append, let's copy the
> file line by line, which ensures all records are properly
> terminated. If we see an extra line, we can simply abort the
> update (there is no point in even copying the rest, as we
> know that it would be identical to the original).