[Git] Add DiffUntracked to render untracked file contents

[KCaverly]

The diff viewer lists untracked files by name in its file index but shows
nothing of what is in them, because git diff HEAD never reports a file git
isn't tracking. For a brand-new file that leaves the most interesting part
invisible — the contents are the whole change.

DiffUntracked diffs each path against os.DevNull with git diff --no-index,
which renders it as a whole-file addition beneath an ordinary diff --git
header. That shape matters: the diff builder indexes those headers for its
file-jump keys and colours the + lines, so new files behave like tracked ones
with no styler changes. Binary files collapse to git's one-line notice rather
than inlining their bytes.

--no-index follows diff(1) and signals "the inputs differ" with exit 1, which
Git treats as failure. Rather than a second runner, Git now delegates to an
internal git() that takes one forgivable exit code; -1 (accept only 0) keeps
every existing caller's behaviour byte-identical. Only an ExitError is
forgiven, so a timeout or a missing binary still fails.

Per-path errors are skipped rather than propagated. An untracked file is by
definition outside git's control and can vanish between the status call that
listed it and this one; losing every other file's diff to that race would be
the wrong trade.

Caller-driven path list: each file costs a subprocess, so the cap belongs
with the caller that knows what it is willing to render.

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com

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Stack (bottom → top):

1. [Git] Add DiffUntracked to render untracked file contents #41 ← this PR
2. [UI] Show uncommitted work as a row in the stack #42
3. [UI] Name one way out of the stack preview #43
4. [UI] Give d the preview and esc the way out #44
5. [UI] Drop the branch-diff jump from the stack screen #45
6. [UI] Stop the stack preview overrunning a narrow terminal #46
7. [Git] Pin the diff flags the body parser depends on #47
8. [UI] Teach both diff viewers the same file-jump key #48
9. [Diff] Thread the [diff] config through the viewer and zoom with z #49
10. [Git] Carry diff args and excludes through every diff call #50
11. [Diff] Add the external diff pager package #51
12. [Config] Accept a [diff] section with an optional pager #52
13. [Docs] Document the diff viewer settings #53