add -p: ignore dirty submodules

Thanks to always running `diff-index` and `diff-files` with the
`--numstat` option (the latter with `--ignore-submodules=dirty`) before
even generating any real diff to parse, the Perl version of `git add -p`
simply ignored dirty submodules and does not even offer them up for
staging.

However, the built-in variant did not use that flag because it tries to
run only one `diff` command, skipping the unneeded
`diff-index`/`diff-files` invocation of the Perl variant and therefore
only faithfully recapitulates what the Perl code does once it _does_
generate and parse the real diff.

This causes a problem when running the built-in `add -p` with
`diff-so-fancy` because that diff colorizer always inserts an empty line
before the diff header to ensure that it produces 4 lines as expected by
`git add -p` (the equivalent of the non-colorized `diff`, `index`, `---`
and `+++` lines). But `git diff-files` does not produce any `index` line
for dirty submodules.

The underlying problem is not even the discrepancy in lines, but that
`git add -p` presents diffs for dirty submodules: there is nothing that
_can_ be staged for those.

Let's fix that bug, and teach the built-in `add -p` to ignore dirty
submodules, too. This _incidentally_ also fixes the `diff-so-fancy`
problem ;-)

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

add-patch.c

@@ -419,7 +419,8 @@ static int parse_diff(struct add_p_state *s, const struct pathspec *ps)
 	}
 	color_arg_index = args.nr;
 	/* Use `--no-color` explicitly, just in case `diff.color = always`. */
-	strvec_pushl(&args, "--no-color", "-p", "--", NULL);
+	strvec_pushl(&args, "--no-color", "--ignore-submodules=dirty", "-p",
+		     "--", NULL);
 	for (i = 0; i < ps->nr; i++)
 		strvec_push(&args, ps->items[i].original);