checkout test: better case-insensitive test on Mac

On Mac OS, `realpath` is deficient in determining the actual filename
on-disk as it will simply provide the string you gave it if that file
exists, instead of returning the filename as it exists.  Instead we
must read the directory entries for the parent directory to get the
canonical filename.

tests/checkout/icase.c

@@ -6,6 +6,8 @@
 
 #ifdef GIT_WIN32
 # include <windows.h>
+#else
+# include <dirent.h>
 #endif
 
 static git_repository *repo;
@@ -40,7 +42,7 @@ void test_checkout_icase__cleanup(void)
 	cl_git_sandbox_cleanup();
 }
 
-static char *test_realpath(const char *in)
+static char *get_filename(const char *in)
 {
 #ifdef GIT_WIN32
 	HANDLE fh;
@@ -65,7 +67,31 @@ static char *test_realpath(const char *in)
 
 	return filename;
 #else
-	return realpath(in, NULL);
+	char *search_dirname, *search_filename, *filename = NULL;
+	git_buf out = GIT_BUF_INIT;
+	DIR *dir;
+	struct dirent *de;
+
+	cl_assert(search_dirname = git_path_dirname(in));
+	cl_assert(search_filename = git_path_basename(in));
+
+	cl_assert(dir = opendir(search_dirname));
+
+	while ((de = readdir(dir))) {
+		if (strcasecmp(de->d_name, search_filename) == 0) {
+			git_buf_join(&out, '/', search_dirname, de->d_name);
+			filename = git_buf_detach(&out);
+			break;
+		}
+	}
+
+	closedir(dir);
+
+	git__free(search_dirname);
+	git__free(search_filename);
+	git_buf_free(&out);
+
+	return filename;
 #endif
 }
 
@@ -74,7 +100,7 @@ static void assert_name_is(const char *expected)
 	char *actual;
 	size_t actual_len, expected_len, start;
 
-	cl_assert(actual = test_realpath(expected));
+	cl_assert(actual = get_filename(expected));
 
 	expected_len = strlen(expected);
 	actual_len = strlen(actual);