fastexport: adjust output to be more canonical
For time zones, git doesn't consider +0 and -0 the same timezone, so use
the former canonically. Add a test case to ensure that non-UTC offsets
are handled correctly. The real name part of the committer name is
normally not quoted, so don't enforce that.
Differential Revision: https://phab.mercurial-scm.org/D8522