git diff -D: omit the preimage of deletes

When reviewing a patch while concentrating primarily on the text after
then change, wading through pages of deleted text involves a cognitive
burden.

Introduce the -D option that omits the preimage text from the patch output
for deleted files.  When used with -B (represent total rewrite as a single
wholesale deletion followed by a single wholesale addition), the preimage
text is also omitted.

To prevent such a patch from being applied by mistake, the output is
designed not to be usable by "git apply" (or GNU "patch"); it is strictly
for human consumption.

It of course is possible to "apply" such a patch by hand, as a human can
read the intention out of such a patch.  It however is impossible to apply
such a patch even manually in reverse, as the whole point of this option
is to omit the information necessary to do so from the output.

Initial request by Mart Sõmermaa, documentation and tests helped by
Michael J Gruber.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 42 insertions, 1 deletions

diff --git a/t/t4022-diff-rewrite.sh b/t/t4022-diff-rewrite.sh
index 2a537a21e8..c00a94b9ba 100755
--- a/t/t4022-diff-rewrite.sh
+++ b/t/t4022-diff-rewrite.sh
@@ -11,7 +11,9 @@ test_expect_success setup '
 	tr \
 	  "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ" \
 	  "nopqrstuvwxyzabcdefghijklmNOPQRSTUVWXYZABCDEFGHIJKLM" \
-	  <"$TEST_DIRECTORY"/../COPYING >test
+	  <"$TEST_DIRECTORY"/../COPYING >test &&
+	echo "to be deleted" >test2 &&
+	git add test2
 
 '
 
@@ -25,5 +27,44 @@ test_expect_success 'detect rewrite' '
 
 '
 
+cat >expect <<EOF
+diff --git a/test2 b/test2
+deleted file mode 100644
+index 4202011..0000000
+--- a/test2
++++ /dev/null
+@@ -1 +0,0 @@
+-to be deleted
+EOF
+test_expect_success 'show deletion diff without -D' '
+
+	rm test2 &&
+	git diff -- test2 >actual &&
+	test_cmp expect actual
+'
+
+cat >expect <<EOF
+diff --git a/test2 b/test2
+deleted file mode 100644
+index 4202011..0000000
+EOF
+test_expect_success 'suppress deletion diff with -D' '
+
+	git diff -D -- test2 >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'show deletion diff with -B' '
+
+	git diff -B -- test >actual &&
+	grep "Linus Torvalds" actual
+'
+
+test_expect_success 'suppress deletion diff with -B -D' '
+
+	git diff -B -D -- test >actual &&
+	grep -v "Linus Torvalds" actual
+'
+
 test_done