Diff() reports renames without DetectRenames() (git's diff.renames default), so it disagrees with Patch() and rename detection can't be turned off

[matt-edmondson]

What's wrong

GitDiffBuilder (GitIntegration/Builders/GitDiffBuilder.cs ~L172-180) adds --find-renames / --find-copies only when requested and never passes --no-renames. Git has defaulted diff.renames=true since 2.9, so rename detection is effectively always on, and its behavior depends on the host's config. GitPatchBuilder already pins this (arguments.Add(_detectRenames ? "--find-renames" : "--no-renames"), the #121 fix), so the two builders now disagree.

Failure scenario (reproduced with a library-level test)

1. Commit old.txt, then git mv old.txt new.txt.
2. repo.Diff().Staged() — no DetectRenames() — returns Renamed old.txt -> new.txt.
3. repo.Patch().Staged() returns Added new.txt; Deleted old.txt.

Consequences:

• DetectRenames() is a no-op in a default git setup; callers cannot turn rename detection off.
• A staging UI that takes its file list from Diff() and hunks from Patch() can't match entries up.
• Results vary by host: diff.renames=false turns renames off; diff.renames=copies emits copy (C) entries without DetectCopies().

Suggested fix

Always set rename detection explicitly, mirroring GitPatchBuilder: emit --find-copies if _detectCopies, else --find-renames if _detectRenames, else --no-renames. Update the DetectRenames()/DetectCopies() XML docs to state detection is off unless requested, regardless of diff.renames (matching IGitPatchBuilder.DetectRenames).

Acceptance criteria

• Diff().Staged() without DetectRenames() reports Added + Deleted for a git mv, matching Patch().
• With DetectRenames() it reports Renamed.
• Tests run with diff.renames set to true, false and copies in the repo config and produce identical results.

[matt-edmondson]

Triage

• Category: Bug
• Priority: Medium. DetectRenames() does nothing on a default git setup, results depend on the host's diff.renames, and Diff() and Patch() disagree. That breaks UIs that pair the file list with hunks. Nothing crashes and no data is lost, so this sits below With log.showSignature=true, Log() throws GitParseException on any signed commit, and Commit() throws after the commit has already been made #135.
• Area / suggested assignee: GitDiffBuilder rename and copy arguments (GitIntegration/Builders/GitDiffBuilder.cs) and the DetectRenames() / DetectCopies() XML docs. Owner: @matt-edmondson
• Duplicates / in progress: not a duplicate. It follows up on GitPatchBuilder doesn't pin context lines or rename detection, so host diff.context=0 yields patches Apply can't stage #121, which pinned this in GitPatchBuilder only, and belongs to the same family as Status() can silently omit untracked files depending on the host's status.showUntrackedFiles config #112 and With log.showSignature=true, Log() throws GitParseException on any signed commit, and Commit() throws after the commit has already been made #135 (user git config leaking into parsed output). No open PR covers it: Read quoted and " b/"-containing paths in Patch() headers #129, [minor] Read a commit with an empty signature name or email #133 and [patch] Keep the patch-fixture -text rule out of the SDK-managed root .gitattributes #134 are unrelated.
• Next step: mirror GitPatchBuilder by always passing one of --find-copies, --find-renames or --no-renames. Add tests that run with diff.renames set to true, false and copies. It is worth auditing the other builders for the same kind of config dependency (status.renames, color.*, diff.noprefix) in the same pass.

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