Allow DiffContextSize to be decreased to zero

[LuxxxLucy]

• PR Description

Per #4012, the diff context size should be able to be decreased to zero. I update the type def for DiffContextSize to be an unsigned integer and add saturated add and subtraction in context_lines_controller.go where the variable is updated (++ or --)

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[jesseduffield]

LGTM, nice work

[LuxxxLucy]

updated with newest commit that should fix the CI complaint.