difftool: fix dir-diff index creation when in a subdirectory

9ec26e7977 (difftool: fix argument handling in subdirs, 2016-07-18)
corrected how path arguments are handled in a subdirectory, but
it introduced a regression in how entries outside of the
subdirectory are handled by dir-diff.

When preparing the right-side of the diff we only include the
changed paths in the temporary area.

The left side of the diff is constructed from a temporary
index that is built from the same set of changed files, but it
was being constructed from within the subdirectory.  This is a
problem because the indexed paths are toplevel-relative, and
thus they were not getting added to the index.

Teach difftool to chdir to the toplevel of the repository before
preparing its temporary indexes.  This ensures that all of the
toplevel-relative paths are valid.

Add test cases to more thoroughly exercise this scenario.

Reported-by: Frank Becker <fb@mooflu.com>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-difftool.perl

@@ -182,6 +182,10 @@ EOF
 		}
 	}
 
+	# Go to the root of the worktree so that the left index files
+	# are properly setup -- the index is toplevel-relative.
+	chdir($workdir);
+
 	# Setup temp directories
 	my $tmpdir = tempdir('git-difftool.XXXXX', CLEANUP => 0, TMPDIR => 1);
 	my $ldir = "$tmpdir/left";

t/t7800-difftool.sh

@@ -374,6 +374,7 @@ test_expect_success PERL 'setup change in subdirectory' '
 	echo master >sub/sub &&
 	git add sub/sub &&
 	git commit -m "added sub/sub" &&
+	git tag v1 &&
 	echo test >>file &&
 	echo test >>sub/sub &&
 	git add file sub/sub &&
@@ -409,12 +410,49 @@ run_dir_diff_test 'difftool --dir-diff ignores --prompt' '
 	grep file output
 '
 
-run_dir_diff_test 'difftool --dir-diff from subdirectory' '
+run_dir_diff_test 'difftool --dir-diff branch from subdirectory' '
 	(
 		cd sub &&
 		git difftool --dir-diff $symlinks --extcmd ls branch >output &&
-		grep sub output &&
-		grep file output
+		# "sub" must only exist in "right"
+		# "file" and "file2" must be listed in both "left" and "right"
+		test "1" = $(grep sub output | wc -l) &&
+		test "2" = $(grep file"$" output | wc -l) &&
+		test "2" = $(grep file2 output | wc -l)
+	)
+'
+
+run_dir_diff_test 'difftool --dir-diff v1 from subdirectory' '
+	(
+		cd sub &&
+		git difftool --dir-diff $symlinks --extcmd ls v1 >output &&
+		# "sub" and "file" exist in both v1 and HEAD.
+		# "file2" is unchanged.
+		test "2" = $(grep sub output | wc -l) &&
+		test "2" = $(grep file output | wc -l) &&
+		test "0" = $(grep file2 output | wc -l)
+	)
+'
+
+run_dir_diff_test 'difftool --dir-diff branch from subdirectory w/ pathspec' '
+	(
+		cd sub &&
+		git difftool --dir-diff $symlinks --extcmd ls branch -- .>output &&
+		# "sub" only exists in "right"
+		# "file" and "file2" must not be listed
+		test "1" = $(grep sub output | wc -l) &&
+		test "0" = $(grep file output | wc -l)
+	)
+'
+
+run_dir_diff_test 'difftool --dir-diff v1 from subdirectory w/ pathspec' '
+	(
+		cd sub &&
+		git difftool --dir-diff $symlinks --extcmd ls v1 -- .>output &&
+		# "sub" exists in v1 and HEAD
+		# "file" is filtered out by the pathspec
+		test "2" = $(grep sub output | wc -l) &&
+		test "0" = $(grep file output | wc -l)
 	)
 '