Render diffs for committed changes, not just uncommitted ones

[wolframarnold]

Fixes #59.

Opening a committed Markdown file's diff — selecting a commit in the Source Control Graph and clicking one of its changed .md files — showed No Markdown changes are available for this file, even though the diff button appeared in the editor title bar.

Why it happened

The refs identifying each side were discarded before any comparison was resolved:

Location	What it did
commandTarget.ts toFileBackedUri	Rewrote a git: URI to its file: path, dropping query.ref. The vscode.Uri branch of getCommandTarget applied that rewrite and hardcoded comparisonHint: "auto", so a commit SHA never survived argument parsing.
gitDiffResolver.ts getComparisonHintFromUris	Recognized only "" (index), "~" (working tree), and HEAD. GitComparisonHint had no value able to carry a commit at all.
extension.ts showDiff	Never read the originalUri / modifiedUri that getCommandTarget already returns — both sides were available and then thrown away.

The comparison fell back to HEAD vs working tree, which for a clean file resolves to cleanHeadToWorkingTree and is rejected by isActionableSingleFileComparison — the message the user saw.

What changed

Deliberately additive: no existing comparison path changes behavior. A comparison is routed to the new code only when a side names a ref that is not "", "~", or HEAD.

• isRevisionRef distinguishes a commit / tag / branch ref from the three well-known refs the existing modes already express. shortenRef and describeComparisonSide build the labels.
• getRevisionComparison detects a comparison naming a revision. getActiveDiffTabUriPair recovers the counterpart revision from the focused diff tab via TabInputTextDiff, because the editor title bar passes only the active resource.
• showRevisionDiff renders the two revisions directly. Content already resolves through the Git extension's content provider (openTextDocument on a git: URI), so only the routing was missing.
• refersToSameFile guards the tab fallback so an unrelated diff tab cannot hijack a command aimed at another file.
• Small cleanups: normalizeFsPath is exported and reused instead of duplicating path normalization, and getComparisonHintFromUris now uses the new ref constants instead of bare "" / "~" literals.

Panel titles read Markdown Diff: example.md (086bdd7 ↔ 010c6f6). Full 40-character SHAs are shortened to 7; branch and tag names are shown as-is. Since both sides of a commit diff share a basename, the existing basename labeling would have produced two identical labels.

This also fixes the quieter half of the bug: when the file had uncommitted edits, the command previously showed the working-tree diff instead of the commit diff that was clicked.

Tests

15 new integration tests in commandTarget.test.ts and gitDiffResolver.test.ts, written before the implementation. Suite goes 22 → 47 passing; test:unit stays at 150. pnpm run pretest is clean.

Coverage: ref classification (commit / tag / branch / HEAD / index / working tree / absent), SHA shortening, side labeling, revision-pair detection including every negative case, same-file matching, and reading both sides of a real diff editor opened with vscode.diff.

Separately, I verified the fix end-to-end against a scratch git repository with two commits: both revisions read correctly through the git content provider, and the command produced a panel titled Markdown Diff: example.md (086bdd7 ↔ 010c6f6). With the source changes reverted and that same check kept, no panel is created at all — confirming it reproduces the reported bug. That check is not included here because the test harness does not open a workspace folder, and adding one to runTest.ts felt out of scope for a bug fix.

Notes for review

• The multi-file diff editor already worked through a different path. When a commit is opened as a multi-file diff, VS Code passes multiDiffEditorOriginalUri / multiFileDiffEditorModifiedUri, which extractComparisonUris already collects — so those land on the argument path rather than the tab fallback. Both are handled.
• canShowRenderedDiff is left as is, and it is still inconsistent. The editor/title menu shows the button via isInDiffEditor, while the context key stays false for a clean committed file. The title-bar button now works, but the right-click editor context menu still will not offer the command there, because its when clause requires the context key. Fixing that means teaching updateCanShowRenderedDiffContext about revision comparisons too — happy to add it here or in a follow-up, whichever you prefer.
• No changelog entry, since entries appear to be written per release — tell me if you would like one added.
• No new user-facing strings, so package.nls.* and the l10n bundles are untouched.