autopoint: Don't risk failing because of user or system git configurations.
Reported by Jamie Kirkpatrick <jkp@kirkconsulting.co.uk>
in <https://savannah.gnu.org/support/?107689>.
* gettext-tools/misc/autopoint.in: Set/unset environment variables, so that git
does not read the user's git configuration nor the system's git configuration.
* gettext-tools/misc/convert-archive.in: Likewise.
