Detect and display renames in `brew update`

Renamed formulae will cease to display as an add/delete pair and instead
end up the in the "Renamed" section. In the future we should be able to
take this information and use it to rename existing kegs during updates,
allowing us to rename formulae without breaking upgrades.

Renaming a formula requires renaming the class, so there will be at
least one add/delete pair in the file. Thus, the similarity threshold
for detecting renames is set at 85% to allow a little bit of content
turnover without losing track of the rename.

Closes Homebrew/homebrew#11158.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>