Re: [GSoC PATCH v3 2/5] pack-write: add helper to fill promisor file after repack

> A ".promisor" file may contain ref names (and their associated hashes)
> that were fetched at the time the corresponding packfile was downloaded.
> This information is used for debugging reasons. This information is
> stored as lines structured like this: "<oid> <ref>".
>
> Create a `copy_promisor_content()` helper function that allows this
> debugging info to not be lost after a `repack`, by coping it inside a new
> ".promisor" file.

> The function logic is the following:
>  * Take all ".promisor" files contained inside the given `repo`.
>  * Ignore those whose name is contained inside the given `strset
>    not_repacked_names`, which basically acts as a "promisor ignorelist"
>    (intended to be used for packfiles that have not been repacked).
>  * Read each line of the remaining ".promisor" files, which can be:
>     * "<oid> <ref>" if the ".promisor" file was never repacked. If so,
>       add the time at which the ".promisor" file was last modified <time>
>       to the line to create the string: "<oid> <ref> <time>".
>     * "<oid> <ref> <time>" if the ".promisor" file was repacked. If so,
>       don't modify it.
>  * Ignore the line if its <oid> is not present inside the
>    "<packtmp>-<dest_hex>.idx" file.
>  * If the destination file "<packtmp>-<dest_hex>.promisor" does not
>    already contain the line, append it to the file.
>
> The function assumes that the contents of all ".promisor" files are
> correctly formed.
>
> The time of last data modification is used in place of the time of file
> creation, because the former is much easier to obtain than the latter
> one.