repack-promisor add helper to fill promisor file after repack

repack-promisor 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 `write_promisor_file_after_repack()` helper function that allows
this debugging info to not be lost after a `repack`, by copying 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 (in Unix time) at which the ".promisor" file was last
      modified <time> to the line, to obtain: "<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 time of last data modification, for never-repacked ".promisor" file,
can be used when comparing the entries in it with entries in another
".promisor" file that did get repacked. With these timestamps, the
debugger will be able to tell at which time the refs at the remote
repository pointed at what object. Also, when looking at already
repacked ".promisor" files, the same ref may appear multiple times, and
having timestamps will help understanding what happened over time.

The function tries its best to deal with malformed ".promisor" files,
ignoring those lines:
 * That cannot be split into "<oid> <ref>" or "<oid> <ref> <time>".
 * Whose <oid> is not a sane hexadecimal string.
 * Whose <ref> does not have the correct format for a refname.

The function `parse_pack_index()`, which is loose in validation, can be
safely used to obtain the `packed_git` of the packs created during the
`repack` because, when `write_promisor_file_after_repack()` is called by
`finish_repacking_promisor_objects()`, we know for a fact that they were
just successfully created by `pack-objects` (also, these packs have not
yet been finalized, and so they are not part of the repository). Anyway,
if an error happens while trying to obtain the `packed_git`, the
".promisor" file will be created empty.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: Tian Yuchen <cat@malon.dev>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: LorenzoPegorari <lorenzo.pegorari2002@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>