[PATCH 2/2] diff-filter: be more careful when looking for negative bits

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

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.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 diff.c         | 8 +++-----
 t/t4202-log.sh | 8 ++++++++
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/diff.c b/diff.c
index c862771a589..fc1151b9c73 100644
--- a/diff.c
+++ b/diff.c
@@ -4821,17 +4821,15 @@ static int diff_opt_diff_filter(const struct option *option,
 	prepare_filter_bits();
 
 	/*
-	 * If there is a negation e.g. 'd' in the input, and we haven't
+	 * If the input starts with a negation e.g. 'd', 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;
+		optch = optarg[0];
+		if (optch >= 'a' && 'z' >= optch) {
 			opt->filter = (1 << (ARRAY_SIZE(diff_status_letters) - 1)) - 1;
 			opt->filter &= ~filter_bit[DIFF_STATUS_FILTER_AON];
-			break;
 		}
 	}
 
diff --git a/t/t4202-log.sh b/t/t4202-log.sh
index 50495598619..28f727937dd 100755
--- a/t/t4202-log.sh
+++ b/t/t4202-log.sh
@@ -142,6 +142,14 @@ test_expect_success 'diff-filter=R' '
 
 '
 
+test_expect_success 'diff-filter=Ra' '
+
+	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
+
+'
+
 test_expect_success 'diff-filter=C' '
 
 	git log -C -C --pretty="format:%s" --diff-filter=C HEAD >actual &&
-- 
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