difftool: use run_command() API in run_file_diff()
Change the run_file_diff() function to use the run_command() API
directly, instead of invoking the run_command_v_opt_cd_env() wrapper.
This allows it, like run_dir_diff(), to use the "args" from "struct
strvec", instead of the "const char **argv" passed into
cmd_difftool(). This will be used in the subsequent commit to get rid
of OPT_ARGUMENT() from cmd_difftool().
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>