Merge branch 'jk/diff-release-filespec-fix'

Running "git diff" while allowing external diff in a state with
unmerged paths used to segfault, which has been corrected.

* jk/diff-release-filespec-fix:
  t7800: simplify difftool test
  diff: allow passing NULL to diff_free_filespec_data()

diff.c

@@ -4115,6 +4115,9 @@ void diff_free_filespec_blob(struct diff_filespec *s)
 
 void diff_free_filespec_data(struct diff_filespec *s)
 {
+	if (!s)
+		return;
+
 	diff_free_filespec_blob(s);
 	FREE_AND_NULL(s->cnt_data);
 }