Squashing broke the commit message when there's code spans in quotes

[programmerjake]

rust-lang/rust#134021 (comment)

bors squash msg="Implement IntoIterator for [&[mut]] Box<[T; N], A>"

gave a squashed commit message of:

Implement  for

Co-authored-by: Mark Rousskov <mark.simulacrum@gmail.com>

[Kobzol]

This is caused by the same underlying problem as #430. Because we use backticks to ignore bors commands, but at the same time sometimes you want to treat backticks as actual text, and not Markdown.