Improve cache sharing between cached markdown fields and mentionables

What does this MR do?

This change makes an item of implicit coupling between the Mentionable and CachedMarkdownField concerns. Mentionable needs to render the fields it depends on to extract references; when those fields are also cached markdown fields, it currently uses that cache to skip the render step.

For the Commit model, we couldn't share the cache because Mentionable was looking at safe_message, while CachedMarkdownField was looking at title, full_title, and description. However, the combination of the latter two is equivalent to safe_message, so we can use them instead.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #244851 (closed)