difftool: avoid $GIT_DIR and $GIT_WORK_TREE

Environment variables are global and hard to reason about.
Use the `--git-dir` and `--work-tree` arguments when invoking `git`
instead of relying on the environment.

Add a test to ensure that difftool's dir-diff feature works when these
variables are present in the environment.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-difftool.perl

@@ -83,20 +83,17 @@ sub changed_files
 {
 	my ($repo_path, $index, $worktree) = @_;
 	$ENV{GIT_INDEX_FILE} = $index;
-	$ENV{GIT_WORK_TREE} = $worktree;
-	my $must_unset_git_dir = 0;
-	if (not defined($ENV{GIT_DIR})) {
-		$must_unset_git_dir = 1;
-		$ENV{GIT_DIR} = $repo_path;
-	}
 
-	my @refreshargs = qw/update-index --really-refresh -q --unmerged/;
-	my @gitargs = qw/diff-files --name-only -z/;
+	my @gitargs = ('--git-dir', $repo_path, '--work-tree', $worktree);
+	my @refreshargs = (
+		@gitargs, 'update-index',
+		'--really-refresh', '-q', '--unmerged');
 	try {
 		Git::command_oneline(@refreshargs);
 	} catch Git::Error::Command with {};
 
-	my $line = Git::command_oneline(@gitargs);
+	my @diffargs = (@gitargs, 'diff-files', '--name-only', '-z');
+	my $line = Git::command_oneline(@diffargs);
 	my @files;
 	if (defined $line) {
 		@files = split('\0', $line);
@@ -105,8 +102,6 @@ sub changed_files
 	}
 
 	delete($ENV{GIT_INDEX_FILE});
-	delete($ENV{GIT_WORK_TREE});
-	delete($ENV{GIT_DIR}) if ($must_unset_git_dir);
 
 	return map { $_ => 1 } @files;
 }
@@ -204,15 +199,6 @@ EOF
 	mkpath($ldir) or exit_cleanup($tmpdir, 1);
 	mkpath($rdir) or exit_cleanup($tmpdir, 1);
 
-	# If $GIT_DIR is not set prior to calling 'git update-index' and
-	# 'git checkout-index', then those commands will fail if difftool
-	# is called from a directory other than the repo root.
-	my $must_unset_git_dir = 0;
-	if (not defined($ENV{GIT_DIR})) {
-		$must_unset_git_dir = 1;
-		$ENV{GIT_DIR} = $repo_path;
-	}
-
 	# Populate the left and right directories based on each index file
 	my ($inpipe, $ctx);
 	$ENV{GIT_INDEX_FILE} = "$tmpdir/lindex";
@@ -241,7 +227,6 @@ EOF
 
 	# If $GIT_DIR was explicitly set just for the update/checkout
 	# commands, then it should be unset before continuing.
-	delete($ENV{GIT_DIR}) if ($must_unset_git_dir);
 	delete($ENV{GIT_INDEX_FILE});
 
 	# Changes in the working tree need special treatment since they are

t/t7800-difftool.sh

@@ -412,6 +412,20 @@ run_dir_diff_test 'difftool --dir-diff from subdirectory' '
 	)
 '
 
+run_dir_diff_test 'difftool --dir-diff from subdirectory with GIT_DIR set' '
+	(
+		GIT_DIR=$(pwd)/.git &&
+		export GIT_DIR &&
+		GIT_WORK_TREE=$(pwd) &&
+		export GIT_WORK_TREE &&
+		cd sub &&
+		git difftool --dir-diff $symlinks --extcmd ls \
+			branch -- sub >output &&
+		grep sub output &&
+		! grep file output
+	)
+'
+
 run_dir_diff_test 'difftool --dir-diff when worktree file is missing' '
 	test_when_finished git reset --hard &&
 	rm file2 &&