Representing timezones with non-integer number of minutes

In a few days, timezones are going to be represented with a git-like [+-]HHMM (and they used to be an integer of minutes).

I think it is an opportunity to add support for precision up to seconds, since we need this to properly support Bazaar: swh/devel/swh-loader-bzr!1 (closed)#inline-50337

Proposed formats:

1. [+-]HHMMSS, which unfortunately is indistinguishable from garbage offsets we can find in ~2k git commits (though it's only a very small fraction of all commits)
2. [+-]HHMM.SS, which may break git (or other implementations) when they try to read it from a vault export (should the vault truncate it?)
3. simply truncate
4. truncate and store seconds somewhere else (like the initial version of swh/devel/swh-loader-bzr!1 (closed) does)

As far as I know, there are no timezones with sub-second precision, but Java seems to represent them as millisecond integer for some reason...

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Migrated from T3886 (view on Phabricator)