list_commits: replace "---" with "--"

GitLab renders "---" as a horizontal line or as a frame. We do not want
this, so replace it with "--".

common.py

@@ -110,7 +110,7 @@ def list_commits(commits):
     """Create a list of commits along with the commit messages"""
     commit_list = ""
     for commit in commits:
-        commit_list += "\n---\n\nCommit: %s\n\n%s" % (commit.web_url, commit.message)
+        commit_list += "\n--\n\nCommit: %s\n\n%s" % (commit.web_url, commit.message)
     return commit_list