Fix incompatibility between repo subdirs and preview mode

course/content.py

@@ -1252,6 +1252,9 @@ def get_course_commit_sha(course, participation):
         preview_sha = participation.preview_git_commit_sha
 
         repo = get_course_repo(course)
+        if isinstance(repo, SubdirRepoWrapper):
+            repo = repo.repo
+
         try:
             repo[preview_sha.encode()]
         except KeyError:

course/utils.py

@@ -421,6 +421,11 @@ class CoursePageContext(object):
             preview_sha = self.participation.preview_git_commit_sha.encode()
 
             repo = get_course_repo(self.course)
+
+            from course.content import SubdirRepoWrapper
+            if isinstance(repo, SubdirRepoWrapper):
+                repo = repo.repo
+
             try:
                 repo[preview_sha]
             except KeyError: