Currently, the first line of a commit message is processed twice by
Commit#all_references, which can be a performance issue if it is very
long. In those cases, `title` returns the first 100 characters of the
line while `description` contains the full line, so this minimises the
amount of text that must be processed.

In cases with a short title and more lines after, i.e., well-formed
commit messages, the first line does not appear in the description, so
we do need both `attr_mentionable`s for references to be scanned.

Changelog: performance