FUSE: multiple origin visits on the same day are ignored

You're probably aware of this, but the current naming scheme doesn't allow showing multiple visits that happened on the same day, which does happen sometimes.

For example:

$ ls <mountpoint>/origin/https%3A%2F%2Fgithub.com%2Fwangnuannuan%2Fembarc_applications
2018-10-12  2018-10-15	2018-10-16  2019-03-08	2019-06-27  2020-03-27

softwareheritage=> select visit, date from origin_visit, origin where origin=origin.id and origin.url='https://github.com/wangnuannuan/embarc_applications';
 visit |             date              
-------+-------------------------------
     1 | 2018-10-12 00:24:31.590759+00
     2 | 2018-10-15 07:48:28.233645+00
     3 | 2018-10-15 13:52:08.767357+00
     4 | 2018-10-15 17:22:29.282168+00
     5 | 2018-10-16 09:04:30.053555+00
     6 | 2019-03-08 20:08:27.763904+00
     7 | 2019-06-27 12:55:15.211531+00
     8 | 2020-03-27 01:03:17.956588+00

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Migrated from T2843 (view on Phabricator)