Add --src-prefix=a/ --dst-prefix=b/ to all git diff/show calls so that
user config (diff.noprefix, diff.mnemonicPrefix) cannot alter the prefix
format the parser depends on. Also add missing ModeCommit test coverage.
When a user has configured a global external diff tool (diff.external /
GIT_EXTERNAL_DIFF) or a textconv filter, git diff/show emit the tool's
output instead of unified diff text. The provider's parser keys off
`diff --git` headers, so it parses zero diffs and the review silently
reports "No files changed".
Add --no-ext-diff --no-textconv to all four git diff/show call sites in
internal/diff/git.go (ModeRange diff, ModeCommit show, and both
workspaceTrackedDiff calls). merge-base and ls-files are left untouched
since they don't run the diff machinery.
Adds an integration test that initializes a real git repo, activates a
garbage GIT_EXTERNAL_DIFF script, and asserts the provider still parses
a non-empty diff (fails before this change, passes after).
Closes#82
AI-assisted contribution.
Introduce gitcmd.Runner with channel-based semaphore to cap concurrent
git subprocesses (default 16, configurable via --max-git-procs). Route
all tool-layer and diff-layer git calls through the shared runner.
Also fix diff.Provider.runGit lacking context.Context — now the full
chain (GetDiff → MergeBase → ParseDiffText → finalizeDiff) propagates
the caller's context for proper cancellation and timeout support.
file_find and finalizeDiff always read from the working tree, even in
range/commit mode where the review targets a specific git ref. This
caused inconsistent file versions compared to file_read and code_search
which correctly used git show. Fix file_find to use git ls-tree and
finalizeDiff to use git show when a ref is specified.
