Handle unborn Git branches when staged changes are hidden.

[am5083]

Fixes #298

When diff-hl-show-staged-changes is nil, diff-hl currently
computes the staged/reference layer with:

git diff-index --cached HEAD

This fails in a repository with an unborn branch because HEAD
does not exist yet. Synchronous updates signal status 128.
Asynchronous updates subsequently pass Git's fatal diagnostic to
diff-hl-changes-from-buffer, which fails in
diff-beginning-of-hunk.

Use git diff --cached [<commit>] instead. When no explicit commit
is supplied, Git compares the index against HEAD when it exists
and against the empty tree on an unborn branch. An explicit
diff-hl-reference-revision is still passed through unchanged.

The regression test creates a temporary unborn repository with
separate staged and worktree changes and verifies both layers with
synchronous and asynchronous updates. The existing test also
checks implicit and explicit HEAD references.

Tested with Emacs 30.2:

• staged-change tests: 3/3 passed
• remaining applicable ERT tests: 15/15 passed
• byte compilation with warnings as errors passed
• git diff --check passed

[dgutov]

Hi! Could you try this version?

(or diff-hl-reference-revision
      (vc-git--rev-parse "HEAD")
      "4b825dc642cb6eb9a060e54bf8d69288fbee4904")

still with diff-index, without switching to git diff. The above literal is the "empty repository" hash for SHA-1, we use it below too.

[dgutov]

Hmm, or maybe not - that adds +1 process call on the common path