--color=debug: combine segments with same labels

[jonathantanmy]

I noticed this when doing the diff whitespace work.

(notice the repeated "diff removed line_number" labels) From

[38;5;1m<<diff removed line_number:: >><<diff removed line_number:: >><<diff removed line_number:: >><<diff removed line_number::1>>[39m<<diff:: >><<diff::    : >>[4m[38;5;1m<<diff removed token::foo>>[24m[39m

to

[38;5;1m<<diff removed line_number::   1>>[39m<<diff::     : >>[4m[38;5;1m<<diff removed token::foo>>[24m[39m

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added tests to cover my changes

[ilyagr]

We discussed this idea with @yuja previously, and he was leaning against it.

However, I still like the idea, since it's so nice to have readable tests. Yuya, if you still would like to have the debug mode to report precisely what happened, would you perhaps be OK with having a separate debug mode for more readable output?

[yuja]

I don't wanna introduce complexity in debug code, which was the main motivation to not deduplicate label outputs. I think this implementation is simple enough to get in. Thanks.