Record the command invocation path early

Record the command invocation path early

We will need the command invocation path in system_path(). This path was
passed to setup_path(), but  system_path() can be called earlier, for
example via:

    main
      commit_pager_choice
        setup_pager
          git_config
            git_etc_gitconfig
              system_path

Therefore, we introduce git_set_argv0_path() and call it as soon as
possible.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>