difftool: add env vars directly in run_file_diff()

difftool: add env vars directly in run_file_diff()

Add the environment variables of the child process directly using
strvec_push() instead of building an array out of them and then adding
that using strvec_pushv().  The new code is shorter and avoids magic
array index values and fragile array padding.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>