UpdateSubmodules().WithDepth(n) makes a full clone when the submodule URL is a local path #279

What's wrong

IGitSubmoduleUpdateBuilder.WithDepth is documented as "Fetches only this many commits of history for a submodule being cloned" (GitIntegration/Builders/GitSubmoduleUpdateBuilder.cs:75-79) and emits --depth <n> (:182). This is the same git behaviour that #251 / #275 fixed for Clone().

When the submodule's URL is a plain local path, git's internal clone ignores the depth. It prints warning: --depth is ignored in local clones; use file:// instead., exits 0, and leaves a full clone. #251 flagged this possibility ("UpdateSubmodules().WithDepth may have the same problem … That hasn't been checked"), but #275 did not change this builder.

You only hit this where git may clone local-path submodules, i.e. with protocol.file.allow=always; since git 2.38.1, git refuses such clones otherwise. That setting is common wherever submodules point at local fixture repositories. This repo's own GitSubmoduleTests sets it (GitIntegration.Test/Integration/GitSubmoduleTests.cs:79).

Reproduction (git 2.43)

super has submodule 'sub' with url /tmp/smx/sub (3 commits); git clone super clone
git -c protocol.file.allow=always submodule update --init --depth 1
  -> warning: --depth is ignored in local clones; use file:// instead.  (exit 0)
git -C sub rev-parse --is-shallow-repository -> false ; rev-list --count HEAD -> 3

Suggested fix

submodule update has no --no-local, so there are two options:

• Rewrite local paths to file:// URLs. Adding -c url."file:///".insteadOf=/ makes the same command produce a shallow clone (verified: is-shallow-repository reports true). This only covers POSIX absolute paths; Windows drive paths would need their own mapping.
• Document the limit. Add a note to WithDepth's remarks that it has no effect for local-path submodule URLs and that callers should use file:// URLs.

Acceptance criteria

• An integration test uses a local-path submodule, sets protocol.file.allow=always, and runs UpdateSubmodules().Initialize().WithDepth(1).
• If the URL rewrite is chosen, the test asserts the submodule is shallow. If the documentation route is chosen, the WithDepth remarks state the limit.