Do not fail if .gitignore is directory

This is designed to fix libgit2sharp #350 where if .gitignore is
a directory we abort all operations that process ignores instead
of just skipping it as core git does.

Also added test that fails without this change and passes with it.

src/attr.c

@@ -278,8 +278,14 @@ static int load_attr_file(
 		return GIT_ENOTFOUND;
 
 	error = git_futils_readbuffer(&content, filename);
-	if (error < 0)
-		return error;
+	if (error < 0) {
+		/* convert error into ENOTFOUND so failed permissions / invalid
+		 * file type don't actually stop the operation in progress.
+		 */
+		return GIT_ENOTFOUND;
+
+		/* TODO: once warnings are available, issue a warning callback */
+	}
 
 	*data = git_buf_detach(&content);
 

tests-clar/attr/ignore.c

 #include "clar_libgit2.h"
+#include "posix.h"
+#include "path.h"
 
 static git_repository *g_repo = NULL;
 
 void test_attr_ignore__initialize(void)
 {
-    g_repo = cl_git_sandbox_init("attr");
+	g_repo = cl_git_sandbox_init("attr");
 }
 
 void test_attr_ignore__cleanup(void)
 {
-    cl_git_sandbox_cleanup();
-    g_repo = NULL;
+	cl_git_sandbox_cleanup();
+	g_repo = NULL;
 }
 
 void assert_is_ignored(bool expected, const char *filepath)
 {
-    int is_ignored;
+	int is_ignored;
 
-    cl_git_pass(git_ignore_path_is_ignored(&is_ignored, g_repo, filepath));
-    cl_assert_equal_i(expected, is_ignored == 1);
+	cl_git_pass(git_ignore_path_is_ignored(&is_ignored, g_repo, filepath));
+	cl_assert_equal_i(expected, is_ignored == 1);
 }
 
 void test_attr_ignore__honor_temporary_rules(void)
 {
-    cl_git_rewritefile("attr/.gitignore", "/NewFolder\n/NewFolder/NewFolder");
+	cl_git_rewritefile("attr/.gitignore", "/NewFolder\n/NewFolder/NewFolder");
 
-    assert_is_ignored(false, "File.txt");
-    assert_is_ignored(true, "NewFolder");
-    assert_is_ignored(true, "NewFolder/NewFolder");
-    assert_is_ignored(true, "NewFolder/NewFolder/File.txt");
+	assert_is_ignored(false, "File.txt");
+	assert_is_ignored(true, "NewFolder");
+	assert_is_ignored(true, "NewFolder/NewFolder");
+	assert_is_ignored(true, "NewFolder/NewFolder/File.txt");
+}
+
+void test_attr_ignore__skip_gitignore_directory(void)
+{
+	cl_git_rewritefile("attr/.git/info/exclude", "/NewFolder\n/NewFolder/NewFolder");
+	p_unlink("attr/.gitignore");
+	cl_assert(!git_path_exists("attr/.gitignore"));
+	p_mkdir("attr/.gitignore", 0777);
+	cl_git_mkfile("attr/.gitignore/garbage.txt", "new_file\n");
+
+	assert_is_ignored(false, "File.txt");
+	assert_is_ignored(true, "NewFolder");
+	assert_is_ignored(true, "NewFolder/NewFolder");
+	assert_is_ignored(true, "NewFolder/NewFolder/File.txt");
 }