[PATCH] git diff -D: omit the preimage of deletes

[Michael J Gruber <git@drmicha.warpmail.net>]

Introduce the -D/--irreversible-delete option which omits the diff for
total deletes. It is similar to -M,-C in its output but irreversible in
the sense that the resulting patch can not be reversed (-R).

When used in connection with -B, omit the diff of the deletion part of a
complete rewrite.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
---
 Documentation/diff-options.txt |   10 +++++++++
 diff.c                         |   14 +++++++++---
 diff.h                         |    1 +
 t/t4022-diff-rewrite.sh        |   43 +++++++++++++++++++++++++++++++++++++++-
 4 files changed, 63 insertions(+), 5 deletions(-)

diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index c93124b..4760d7a 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -259,6 +259,16 @@ endif::git-log[]
 	projects, so use it with caution.  Giving more than one
 	`-C` option has the same effect.
 
+-D::
+--irreversible-delete::
+	Omit the preimage for deletes, i.e. print only the header but not
+	the diff between the preimage and `/dev/null`. The resulting patch
+	is irreversible in the sense that it can not be applied in reverse
+	(-R).
++
+When used together with `-B`, omit also the preimage in the deletion part
+of a delete/create pair.
+
 -l<num>::
 	The `-M` and `-C` options require O(n^2) processing time where n
 	is the number of potential rename/copy targets.  This
diff --git a/diff.c b/diff.c
index 5422c43..9ea1de1 100644
--- a/diff.c
+++ b/diff.c
@@ -1943,7 +1943,11 @@ static void builtin_diff(const char *name_a,
 		}
 	}
 
-	if (!DIFF_OPT_TST(o, TEXT) &&
+	if (o->irreversible_delete && lbl[1][0] == '/') {
+		fprintf(o->file, "%s", header.buf);
+		strbuf_reset(&header);
+		goto free_ab_and_return;
+	} else if (!DIFF_OPT_TST(o, TEXT) &&
 	    ( (!textconv_one && diff_filespec_is_binary(one)) ||
 	      (!textconv_two && diff_filespec_is_binary(two)) )) {
 		if (fill_mmfile(&mf1, one) < 0 || fill_mmfile(&mf2, two) < 0)
@@ -1963,8 +1967,7 @@ static void builtin_diff(const char *name_a,
 			fprintf(o->file, "%sBinary files %s and %s differ\n",
 				line_prefix, lbl[0], lbl[1]);
 		o->found_changes = 1;
-	}
-	else {
+	} else {
 		/* Crazy xdl interfaces.. */
 		const char *diffopts = getenv("GIT_DIFF_OPTS");
 		xpparam_t xpp;
@@ -3160,6 +3163,9 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 			return error("invalid argument to -M: %s", arg+2);
 		options->detect_rename = DIFF_DETECT_RENAME;
 	}
+	else if (!strcmp(arg, "-D") || !strcmp(arg, "--irreversible-delete")) {
+		options->irreversible_delete = 1;
+	}
 	else if (!prefixcmp(arg, "-C") || !prefixcmp(arg, "--find-copies=") ||
 		 !strcmp(arg, "--find-copies")) {
 		if (options->detect_rename == DIFF_DETECT_COPY)
@@ -4205,7 +4211,7 @@ void diffcore_std(struct diff_options *options)
 			diffcore_break(options->break_opt);
 		if (options->detect_rename)
 			diffcore_rename(options);
-		if (options->break_opt != -1)
+		if (options->break_opt != -1 && !options->irreversible_delete)
 			diffcore_merge_broken();
 	}
 	if (options->pickaxe)
diff --git a/diff.h b/diff.h
index 310bd6b..11d13cf 100644
--- a/diff.h
+++ b/diff.h
@@ -104,6 +104,7 @@ struct diff_options {
 	int interhunkcontext;
 	int break_opt;
 	int detect_rename;
+	int irreversible_delete;
 	int skip_stat_unmatch;
 	int line_termination;
 	int output_format;
diff --git a/t/t4022-diff-rewrite.sh b/t/t4022-diff-rewrite.sh
index 2a537a2..c00a94b 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
 
-- 
1.7.4.2.668.gba03a4