shell: add missing initialization of argv0_path

shell: add missing initialization of argv0_path

According to c6dfb39 (remote-curl: add missing initialization of
argv0_path, 2009-10-13), stand-alone programs (non-builtins)
must call git_extract_argv0_path(argv[0]) in order to help builds
that derive the installation prefix at runtime. Without this call,
the program segfaults (or raises an assertion failure).

Signed-off-by: Dima Sharov <git.avalakvista@gmail.com>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>