Add color diffs to htmldocck.py

[jyn514]

Currently, test failures look like this:

---- [rustdoc] rustdoc/where.rs stdout ----

error: htmldocck failed!
status: exit code: 1
command: "/usr/bin/python" "/home/joshua/rustc/src/etc/htmldocck.py" "/home/joshua/rustc/build/x86_64-unknown-linux-gnu/test/rustdoc/where" "/home/joshua/rustc/src/test/rustdoc/where.rs"
stdout:
------------------------------------------

------------------------------------------
stderr:
------------------------------------------
7: @has check failed
	`XPATH PATTERN` did not match
	// @has foo/trait.Bravo.html '//pre' "pub trait Bravo<B> where B: MyTrait"

Encountered 1 errors

This tells you what test went wrong, but not where. Why didn't the pattern match? No idea.

Instead, htmldocck could tell you exactly what went wrong:

I hacked this together with some scripts, I'll see about turning it into something maintainable. Giant thank you to @Nemo157 for the help!

Originally posted by @jyn514 in #77820 (comment)

[jyn514]

So I don't lose it, these are the hacky scripts:

./x.py test src/test/rustdoc 2>&1 | grep '^    \[rustdoc\] ' | awk '{print $2}' | xargs printf "src/test/%s\n" | xargs -n1 diff-toolchain nightly stage1 >differences.log

where diff-toolchain is

#!/bin/sh

set -eu

if ! [ $# = 3 ]; then
	echo "usage: $0 <before> <after> <file>"
	exit 1
fi

before=$1
after=$2
file="$3"

rustdoc +$before "$file"
tidy-rustdoc doc doc-$before

rustdoc +$after "$file"
tidy-rustdoc doc doc-$after

diff -ur doc-$before doc-$after | delta

and tidy-rustdoc is Nemo157/dotfiles@0a3f39e.