main: re-exec self with the current interp

The launcher already raised itself up to use Python 3 on the fly, and
the main.py script uses a plain `python` shebang.  So make sure we use
the active interpreter when re-execing ourselves to avoid falling back
down to Python 2 (which then triggers warnings).

Change-Id: Ic53c07dead3bc9233e4089a0a422f83bb5ac2f91
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/263272
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@digital.ai>