fix(p4): fix output parsing bugs and add VCS adapter tests

[dlyongemallo]

No description provided.

[Copilot]

Pull request overview

This PR improves Diffview’s Perforce adapter output parsing (opened/diff/diff2/describe) and adds functional coverage for Perforce, Jujutsu, and Mercurial adapters, with CI updates to install required CLIs.

Changes:

• Fix Perforce p4 opened, p4 diff -sl, and p4 diff2 -ds parsing to avoid revision-suffix issues and better detect adds/deletes/modifies.
• Adjust p4 describe parsing to correctly handle blank lines in the “Affected files …” section.
• Add/extend functional tests for p4/jj/hg adapters and update CI + README to reflect tool requirements.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
lua/diffview/vcs/adapters/p4/init.lua	Updates Perforce tracked-files parsing for opened, diff -sl, and diff2 -ds.
lua/diffview/vcs/adapters/p4/commit.lua	Refines p4 describe parsing termination logic to handle blank lines.
lua/diffview/tests/functional/p4_adapter_spec.lua	Adds Perforce adapter functional + parsing tests using a disposable p4d repo.
lua/diffview/tests/functional/jj_adapter_spec.lua	Adds Jujutsu integration tests for tracked files/show behavior.
lua/diffview/tests/functional/hg_adapter_spec.lua	Adds Mercurial functional tests for tracked files/show behavior.
README.md	Documents minimum Perforce CLI version and environment prerequisites.
.github/workflows/ci.yml	Downloads/caches p4/p4d and jj so integration tests can run in CI.

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