Keep the .git directory in the local course build testing setup

Since `git clean` is run at the end of the course build nowadays,
it caused a crash in the local testing setup when
`settings.LOCAL_COURSE_SOURCE_PATH` is used.
It crashed because the course directory was missing the `.git`
directory and thus, it was not a git repository.
The build error was the following:

```
stdout: fatal: not a git repository (or any parent up to mount point /tmp)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set)
```

Fix the crash by including the `.git` directory when
the course source directory is copied to the build directory.
The `.git` directory used to be excluded simply because the copy
operation is a bit faster without it.

builder/builder.py

@@ -571,14 +571,8 @@ def build_course(
             path = CourseConfig.local_source_path_to(course_key)
             build_logger.debug(f"Course origin not set: copying the course sources from {path} to the build directory.")
 
-            def ignore_func(directory, contents):
-                # Do not copy the .git directory in shutil.copytree().
-                if '.git' in contents:
-                    return ('.git',)
-                return ()
-
             shutil.rmtree(build_path, ignore_errors=True)
-            shutil.copytree(path, build_path, symlinks=True, ignore=ignore_func)
+            shutil.copytree(path, build_path, symlinks=True)
         else:
             build_logger.warning(f"Course origin not set: skipping git update\n")