diff: respect textconv in rewrite diffs

Currently we just skip rewrite diffs for binary files; this
patch makes an exception for files which will be textconv'd,
and actually performs the textconv before generating the
diff.

Conceptually, rewrite diffs should be in the exact same
format as the a non-rewrite diff, except that we refuse to
share any context. Thus it makes very little sense for "git
diff" to show a textconv'd diff, but for "git diff -B" to
show "Binary files differ".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4031-diff-rewrite-binary.sh

@@ -7,6 +7,8 @@ test_description='rewrite diff on binary file'
 # We must be large enough to meet the MINIMUM_BREAK_SIZE
 # requirement.
 make_file() {
+	# common first line to help identify rewrite versus regular diff
+	printf "=\n" >file
 	for i in 1 2 3 4 5 6 7 8 9 10
 	do
 		for j in 1 2 3 4 5 6 7 8 9
@@ -16,7 +18,7 @@ make_file() {
 				printf "$1\n"
 			done
 		done
-	done >file
+	done >>file
 }
 
 test_expect_success 'create binary file with changes' '
@@ -42,4 +44,24 @@ test_expect_success 'rewrite diff can show binary patch' '
 	grep "GIT binary patch" diff
 '
 
+{
+	echo "#!$SHELL_PATH"
+	cat >dump <<'EOF'
+perl -e '$/ = undef; $_ = <>; s/./ord($&)/ge; print $_' < "$1"
+EOF
+} >dump
+chmod +x dump
+
+test_expect_success 'setup textconv' '
+	echo file diff=foo >.gitattributes &&
+	git config diff.foo.textconv "$PWD"/dump
+'
+
+test_expect_success 'rewrite diff respects textconv' '
+	git diff -B >diff &&
+	grep "dissimilarity index" diff &&
+	grep "^-61" diff &&
+	grep "^-0" diff
+'
+
 test_done