submodule.c: migrate diff output to use emit_diff_symbol

submodule.c: migrate diff output to use emit_diff_symbol

As the submodule process is no longer attached to the same file pointer
'o->file' as the superprojects process, there is a different result in
color.c::check_auto_color. That is why we need to pass coloring explicitly,
such that the submodule coloring decision will be made by the child process
processing the submodule. Only DIFF_SYMBOL_SUBMODULE_PIPETHROUGH contains
color, the other symbols are for embedding the submodule output into the
superprojects output.

Remove the colors from the function signatures, as all the coloring
decisions will be made either inside the child process or the final
emit_diff_symbol, but not in the functions driving the submodule diff.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>