diff: shortcut for diff'ing two binary SHA-1 objects

If we are given two SHA-1 and asked to determine if they are different
(but not _what_ differences), we know right away by comparing SHA-1.

A side effect of this patch is, because large files are marked binary,
diff-tree will not need to unpack them. 'diff-index --cached' will not
either. But 'diff-files' still does.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -2324,6 +2324,19 @@ static void builtin_diff(const char *name_a,
 	} else if (!DIFF_OPT_TST(o, TEXT) &&
 	    ( (!textconv_one && diff_filespec_is_binary(one)) ||
 	      (!textconv_two && diff_filespec_is_binary(two)) )) {
+		if (!one->data && !two->data &&
+		    S_ISREG(one->mode) && S_ISREG(two->mode) &&
+		    !DIFF_OPT_TST(o, BINARY)) {
+			if (!hashcmp(one->sha1, two->sha1)) {
+				if (must_show_header)
+					fprintf(o->file, "%s", header.buf);
+				goto free_ab_and_return;
+			}
+			fprintf(o->file, "%s", header.buf);
+			fprintf(o->file, "%sBinary files %s and %s differ\n",
+				line_prefix, lbl[0], lbl[1]);
+			goto free_ab_and_return;
+		}
 		if (fill_mmfile(&mf1, one) < 0 || fill_mmfile(&mf2, two) < 0)
 			die("unable to read files to diff");
 		/* Quite common confusing case */

t/t1050-large.sh

@@ -116,6 +116,16 @@ test_expect_success 'diff --stat' '
 	git diff --stat HEAD^ HEAD
 '
 
+test_expect_success 'diff' '
+	git diff HEAD^ HEAD >actual &&
+	grep "Binary files.*differ" actual
+'
+
+test_expect_success 'diff --cached' '
+	git diff --cached HEAD^ >actual &&
+	grep "Binary files.*differ" actual
+'
+
 test_expect_success 'hash-object' '
 	git hash-object large1
 '