feat: `importdemocourse --repo-dir=...` option

This allows us to run:

    tutor local do importdemocourse --repo=https://github.com/openedx/openedx-test-course --version=open-release/palm.master --repo-dir=test-course/course

changelog.d/20230502_090803_regis_palm.md

@@ -0,0 +1,2 @@
+- [Feature] Add the `do importdemocourse --repo-dir=...` option, to import courses from subdirectories of git repositories. This allows us to import the openedx-test-course in Palm with: `tutor local do importdemocourse --repo=https://github.com/openedx/openedx-test-course --version=o
+pen-release/palm.master --repo-dir=test-course/course`. (by @regisb)

tutor/commands/jobs.py

@@ -146,19 +146,26 @@ u.save()"
     show_default=True,
     help="Git repository that contains the course to be imported",
 )
+@click.option(
+    "-d",
+    "--repo-dir",
+    default="",
+    show_default=True,
+    help="Git relative subdirectory to import data from",
+)
 @click.option(
     "-v",
     "--version",
     help="Git branch, tag or sha1 identifier. If unspecified, will default to the value of the OPENEDX_COMMON_VERSION setting.",
 )
 def importdemocourse(
-    repo: str, version: t.Optional[str]
+    repo: str, repo_dir: str, version: t.Optional[str]
 ) -> t.Iterable[tuple[str, str]]:
     version = version or "{{ OPENEDX_COMMON_VERSION }}"
     template = f"""
 # Import demo course
 git clone {repo} --branch {version} --depth 1 /tmp/course
-python ./manage.py cms import ../data /tmp/course
+python ./manage.py cms import ../data /tmp/course/{repo_dir}
 
 # Re-index courses
 ./manage.py cms reindex_course --all --setup"""