plumbing: diff, add colored output support

[twpayne]

Fixes #33.

This is a draft PR. Please tell me if it more-or-less follows the project's coding style and what changes are likely to be needed to make it more acceptable.

At the moment, it only supports the default git diff colors as defined by git 2.26.2. It should be extensible to reading custom colors from a git config file, if needed.

Before merging, this needs a few tweaks to ensure the output matches git's as far as reasonably possible.

[twpayne]

cc @mcuadros

[twpayne]

It looks like the CI failures are unrelated to the changes in this PR.

[mcuadros]

Looks promising, just remember that should be optional.
Thanks

[twpayne]

Looks promising, just remember that should be optional.

Yes, it's optional.

The API for creating and using a UnifiedEncoder is unchanged:

ue := NewUnifiedEncoder(w, ctxtLines)

To enable color, you need to explicitly call UnifiedEncoder.SetColor to set a ColorConfig:

ue.SetColor(NewColorConfig()) // Use default git colors.

NewColorConfig returns the default colored config. You can also pass nil to disable colors (which is the default):

ue.SetColor(nil) // Disable colors.

[twpayne]

This is now ready for review. Key points:

• The API to enable color is optional and off by default. Existing code will continue to behave exactly as before.
• I added a new plumbing/color package for functionality that exists in git's color.c and color.h files in the git source code.
• The colorized diffs are sensible and match git's output given go-git's existing diff code.

The CI failure seems unrelated to the change in this PR.

Many thanks for any review :)

[mcuadros]

can we move this to internal?

[twpayne]

Done.

[twpayne]

Thanks for the fast review and merge :)

[mcuadros]

Thanks for the contribution :D