Comments: can't use `>` shorthand to create a blockquote in the first line of a comment

**Steps to repeat**
1. Make a comment like: `> This is a blockquote.` where that is the first line of the comment.
2. Save.

**What I expected to happen:**
Display as a blockquote after saving.

**What happened instead:**
The end result displays as plain text.

**Screenshots:**

Raised by @kjellr

No file or test is named in the issue. Start by reproducing the problem with a comment whose first line is `> This is a blockquote.`, then trace the comment parsing or rendering entry point; done means the saved first-line shorthand displays as a blockquote.

Written by the indexing model from the issue text.