YAML frontmatter cannot have a folded scalar

[SirNate0]

Describe the bug
Including a folded scalar in the YAML results in an error, "Failed to load YAML frontmatter: (<unknown>): did not find expected alphabetic or numeric character while scanning an anchor at...".

To Reproduce
Add front matter such as the following

---
Literal Scalar: |
  abc
  
  123
Folded Scalar: >
  abc
  
  123
Escaped: "abc\n\n123"
---

You will get as the output

errors
Failed to load YAML frontmatter: (<unknown>): did not find expected alphabetic or numeric character while scanning an anchor at line 6 column 16

Expected behavior
The yaml should be parsed successfully. The folded scalar should have different whitespace than the others (only one newline instead of two), though keeping the present behavior where new lines seem to be turned into spaces is acceptable, I suppose, as long as it doesn't give an error.

Environment Info

Gollum 5.3.2
Running on: x86_64-linux-gnu with Ruby version 3.0.2
Using:
rugged 1.7.1
gollum-rugged_adapter 2.1.0
gollum-lib 5.2.4
Markdown rendering gem: kramdown
Other renderering gems:
none

[dometto]

Thanks for the report. I was able to reproduce and write a test case. The problem was we were HTML escaping the entire YAML string before loading it, so the > was getting scrubbed, invalidating the YAML.

See gollum/gollum-lib#454 for the test case and fix.

Instead of trying to handle this one exception, I thought it might be better to go for a robust approach and not touch the YAML before we parse it. I think the new approach is robust and secure, but of course it will be slower. Feedback very welcome!