RepoCommand: record manifest shallow recommendation in .gitmodules

RepoCommand: record manifest shallow recommendation in .gitmodules

Git core learned about the submodule.<name>.shallow option in
.gitmodules files, which is a recommendation to clone a submodule
shallow. A repo manifest may record a clone depth recommendation as
an optional field, which contains more information than a binary
shallow/nonshallow recommendation, so any attempted conversion may be
lossy. In practice the clone depth recommendation is either '1' or doesn't
exist, which is the binary behavior we have in Git core.

Change-Id: I51aa9cb6d1d9660dae6ab6d21ad7bae9bc5325e6
Signed-off-by: Stefan Beller <sbeller@google.com>