Mishandling of fractional seconds in ISO 8601 format

git-commit(1) claims we support ISO 8601 format. However, our approxidate code misparses some dates with fractional seconds.

Reproduction:

  git init foo
  cd foo
  echo abc > abc.txt
  git add .
  git commit --date="2020-04-03T12:43:55.019-04:00”

This should produce Fri Apr 3 12:43:55 2020 -0400, but actually produces Sun Apr 19 12:43:55 2020 +0000 (at least on my system, which uses UTC). Note the different date, which is 16 days away from what is expected.

Since we claim to support ISO 8601, we need to either reject fractional seconds with an error, or accept and ignore them. If what we really support is RFC 3339 (which I suspect it is), we need to do the latter, since that profile explicitly permits them in the syntax, as well as update the documentation accordingly.

This was originally reported at https://stackoverflow.com/questions/61193896/how-does-git-parse-date-string/61197722. I don't plan to send a patch for this right now, but I wanted to make sure it was reported to the list.