Status() and Diff() throw GitRepositoryNotFoundException for a valid repository when one submodule's git directory is missing #238

What's wrong

GitCommandBuilder.CreateException classifies any failure whose diagnostic contains not a git repository as GitRepositoryNotFoundException, documented as "The path given is not inside a git working tree":

git uses the same phrase for a submodule's broken gitfile, for example fatal: not a git repository: sub/../.git/modules/sub. In that case the superproject is a perfectly good repository, but the library tells the caller it is not one.

Reproduction (git 2.43, verified through the library)

git init up && git -C up commit --allow-empty -m base
git clone up down
git -c protocol.file.allow=always -C down submodule add ../up sub
git -C down commit -m sub
rm -rf down/.git/modules/sub      # e.g. a copy or bind mount that left out .git/modules

Then, against down:

OpenAsync(down)  -> succeeds (rev-parse does not touch submodules)
Status()         -> GitRepositoryNotFoundException: git exited with code 128: fatal: not a git repository: sub/../.git/modules/sub
Diff()           -> GitRepositoryNotFoundException: (same)
Submodules()     -> ok
Log()            -> ok

Why it matters

The same GitRepository is reported as found by OpenAsync and as not found by Status(). A caller that does the obvious thing with GitRepositoryNotFoundException (show "this folder is not a git repository", offer Init, drop it from a list of repositories) acts on a false statement, and the real problem, one broken submodule, is hidden behind the wrong type.

Suggested fix

Only classify as GitRepositoryNotFoundException when git is talking about the repository the command was scoped to:

• the discovery failure forms, not a git repository (or any of the parent directories) and not a git repository (or any parent up to mount point …), or
• a not a git repository: <path> whose path is the repository's own git directory (for example a linked worktree whose admin directory has gone).

Any other not a git repository: <path>, such as one naming …/.git/modules/…, should stay a plain GitCommandException carrying git's message.

Acceptance criteria

• With the reproduction above, Status() and Diff() throw GitCommandException and not GitRepositoryNotFoundException, and the message still names the broken submodule path.
• Running a verb against a directory that is not inside any repository still throws GitRepositoryNotFoundException.
• A unit test on CreateException covers both diagnostic forms.