[PATCH] difftool: gracefully handle symlinks to directories

[David Aguilar <davvid@gmail.com>]

difftool's dir-diff feature was blindly feeding worktree paths
to hash-object without checking whether the path was indeed a
file, causing the feature to fail when repositories contain
symlinks to directories.

Ensure that only files are ever given to hash-object.
Add a test to demonstrate the breakage.

Reported-by: Ismail Badawi <ismail@badawi.io>
Signed-off-by: David Aguilar <davvid@gmail.com>
---
 git-difftool.perl   |  4 +---
 t/t7800-difftool.sh | 19 +++++++++++++++++++
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/git-difftool.perl b/git-difftool.perl
index 7df7c8a..1abe647 100755
--- a/git-difftool.perl
+++ b/git-difftool.perl
@@ -70,9 +70,7 @@ sub use_wt_file
 	my ($repo, $workdir, $file, $sha1) = @_;
 	my $null_sha1 = '0' x 40;
 
-	if (! -e "$workdir/$file") {
-		# If the file doesn't exist in the working tree, we cannot
-		# use it.
+	if (! -f "$workdir/$file") {
 		return (0, $null_sha1);
 	}
 
diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh
index 48c6e2b..ec8bc8c 100755
--- a/t/t7800-difftool.sh
+++ b/t/t7800-difftool.sh
@@ -504,4 +504,23 @@ test_expect_success PERL 'difftool properly honors gitlink and core.worktree' '
 	)
 '
 
+test_expect_success PERL,SYMLINKS 'difftool --dir-diff symlinked directories' '
+	git init dirlinks &&
+	(
+		cd dirlinks &&
+		git config diff.tool checktrees &&
+		git config difftool.checktrees.cmd "echo good" &&
+		mkdir foo &&
+		: >foo/bar &&
+		git add foo/bar &&
+		test_commit symlink-one &&
+		ln -s foo link &&
+		git add link &&
+		test_commit symlink-two &&
+		echo good >expect &&
+		git difftool --tool=checktrees --dir-diff HEAD~ >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done
-- 
2.6.2.281.gac28444