difftool: teach difftool to handle directory diffs

When 'difftool' is called to compare a range of commits that modify
more than one file, it opens a separate instance of the diff tool for
each file that changed.

The new '--dir-diff' option copies all the modified files to a temporary
location and runs a directory diff on them in a single instance of the
diff tool.

Signed-off-by: Tim Henigan <tim.henigan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh
index e716d06..478c1be 100755
--- a/t/t7800-difftool.sh
+++ b/t/t7800-difftool.sh
@@ -319,4 +319,43 @@
 	echo "$diff" | stdin_doesnot_contain br2
 '
 
+test_expect_success PERL 'setup change in subdirectory' '
+	git checkout master &&
+	mkdir sub &&
+	echo master >sub/sub &&
+	git add sub/sub &&
+	git commit -m "added sub/sub" &&
+	echo test >>file &&
+	echo test >>sub/sub &&
+	git add . &&
+	git commit -m "modified both"
+'
+
+test_expect_success PERL 'difftool -d' '
+	diff=$(git difftool -d --extcmd ls branch) &&
+	echo "$diff" | stdin_contains sub &&
+	echo "$diff" | stdin_contains file
+'
+
+test_expect_success PERL 'difftool --dir-diff' '
+	diff=$(git difftool --dir-diff --extcmd ls branch) &&
+	echo "$diff" | stdin_contains sub &&
+	echo "$diff" | stdin_contains file
+'
+
+test_expect_success PERL 'difftool --dir-diff ignores --prompt' '
+	diff=$(git difftool --dir-diff --prompt --extcmd ls branch) &&
+	echo "$diff" | stdin_contains sub &&
+	echo "$diff" | stdin_contains file
+'
+
+test_expect_success PERL 'difftool --dir-diff from subdirectory' '
+	(
+		cd sub &&
+		diff=$(git difftool --dir-diff --extcmd ls branch) &&
+		echo "$diff" | stdin_contains sub &&
+		echo "$diff" | stdin_contains file
+	)
+'
+
 test_done