Add shallow-clone (depth) support to GitFetcher

[stanhu]

Description

Adds a depth source option to git sources so components can be fetched with a shallow clone instead of downloading the full repository history. This can significantly cut fetch time and disk usage for large repositories where full history isn't needed.

Behavior

When a git source sets a positive integer depth:

source git: "https://example.com/repo.git", depth: 1

the fetcher initializes an empty repo and fetches the resolved revision (SHA) directly at that depth, instead of git cloneing the full history:

git init --quiet .
git fetch --depth N <url> <sha>

Fetching the resolved SHA (rather than a branch/tag ref) means shallow mode:

• works for tag, branch, and SHA pins alike, and
• is immune to a pinned branch tip moving between resolution and fetch.

A missing or non-positive depth preserves the existing full-clone behavior, so this is fully backward compatible.

Submodules

In shallow mode there is no clone --recursive, so git_checkout uses submodule update --init --recursive. It deliberately does not pass --depth: a shallow submodule fetch targets the submodule's branch tip and fails when the pinned submodule commit isn't near it, so submodules are fetched at full depth.

Requirements / caveats

• The remote must allow fetching a reachable SHA (uploadpack.allowReachableSHA1InWant / allowAnySHA1InWant).
• Components whose build depends on git history (e.g. git describe) should not use this option.

Changes

• lib/omnibus/fetchers/git_fetcher.rb — add clone_depth; git_clone does init + git_fetch in shallow mode; git_fetch fetches the resolved SHA with --depth; git_checkout initializes submodules at full depth.
• lib/omnibus/software.rb — allow :depth as a valid git source key and document the option.
• spec/unit/fetchers/git_fetcher_spec.rb — specs covering no/zero/positive depth, larger depth, submodule handling in both modes, and the fetch-resolved-SHA behavior.

Testing

bundle exec rspec spec/unit/fetchers/git_fetcher_spec.rb — 21 examples, 0 failures. chefstyle clean on all changed files.

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