diff-filter: be more careful when looking for negative bits

The `--diff-filter=<bits>` option allows to filter the diff by certain
criteria, for example `R` to only show renamed files. It also supports
negating a filter via a down-cased letter, i.e. `r` to show _everything
but_ renamed files.

However, the code is a bit overzealous when trying to figure out whether
`git diff` should start with all diff-filters turned on because the user
provided a lower-case letter: if the `--diff-filter` argument starts
with an upper-case letter, we must not start with all bits turned on.

Even worse, it is possible to specify the diff filters in multiple,
separate options, e.g. `--diff-filter=AM [...] --diff-filter=m`.

Let's accumulate the include/exclude filters independently, and only
special-case the "only exclude filters were specified" case after
parsing the options altogether.

Note: The code replaced by this commit took pains to avoid setting any
unused bits of `options->filter`. That was unnecessary, though, as all
accesses happen via the `filter_bit_tst()` function using specific bits,
and setting the unused bits has no effect. Therefore, we can simplify
the code by using `~0` (or in this instance, `~<unwanted-bit>`).

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

diff.c

@@ -4720,6 +4720,12 @@ void diff_setup_done(struct diff_options *options)
 	if (!options->use_color || external_diff())
 		options->color_moved = 0;
 
+	if (options->filter_not) {
+		if (!options->filter)
+			options->filter = ~filter_bit[DIFF_STATUS_FILTER_AON];
+		options->filter &= ~options->filter_not;
+	}
+
 	FREE_AND_NULL(options->parseopts);
 }
 
@@ -4820,21 +4826,6 @@ static int diff_opt_diff_filter(const struct option *option,
 	BUG_ON_OPT_NEG(unset);
 	prepare_filter_bits();
 
-	/*
-	 * If there is a negation e.g. 'd' in the input, and we haven't
-	 * initialized the filter field with another --diff-filter, start
-	 * from full set of bits, except for AON.
-	 */
-	if (!opt->filter) {
-		for (i = 0; (optch = optarg[i]) != '\0'; i++) {
-			if (optch < 'a' || 'z' < optch)
-				continue;
-			opt->filter = (1 << (ARRAY_SIZE(diff_status_letters) - 1)) - 1;
-			opt->filter &= ~filter_bit[DIFF_STATUS_FILTER_AON];
-			break;
-		}
-	}
-
 	for (i = 0; (optch = optarg[i]) != '\0'; i++) {
 		unsigned int bit;
 		int negate;
@@ -4851,7 +4842,7 @@ static int diff_opt_diff_filter(const struct option *option,
 			return error(_("unknown change class '%c' in --diff-filter=%s"),
 				     optarg[i], optarg);
 		if (negate)
-			opt->filter &= ~bit;
+			opt->filter_not |= bit;
 		else
 			opt->filter |= bit;
 	}

diff.h

@@ -283,7 +283,7 @@ struct diff_options {
 	struct diff_flags flags;
 
 	/* diff-filter bits */
-	unsigned int filter;
+	unsigned int filter, filter_not;
 
 	int use_color;
 

t/t4202-log.sh

@@ -142,6 +142,19 @@ test_expect_success 'diff-filter=R' '
 
 '
 
+test_expect_success 'multiple --diff-filter bits' '
+
+	git log -M --pretty="format:%s" --diff-filter=R HEAD >expect &&
+	git log -M --pretty="format:%s" --diff-filter=Ra HEAD >actual &&
+	test_cmp expect actual &&
+	git log -M --pretty="format:%s" --diff-filter=aR HEAD >actual &&
+	test_cmp expect actual &&
+	git log -M --pretty="format:%s" \
+		--diff-filter=a --diff-filter=R HEAD >actual &&
+	test_cmp expect actual
+
+'
+
 test_expect_success 'diff-filter=C' '
 
 	git log -C -C --pretty="format:%s" --diff-filter=C HEAD >actual &&