Sync: Fix full submodule sync while shallow specified

Git allows to clone submodules as shallow clone [1]. On the other
hand, when repo synchronize a projcet with submodules inside, it
ignores the shallow parameter.

When a project contains submodules, project.py parses the .gitmodules
file for URL and path. This parsing does not consider the shallow
option. Consequently, this parameter is not propgated to newly
created Project instance for that submodule.

[1] https://git-scm.com/docs/gitmodules#Documentation/gitmodules.txt-submoduleltnamegtshallow

Change-Id: I54fc9c69ae1b8e3cda2801202e3f0c7693b718d2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/454261


Tested-by: Albert Akmukhametov <alb.02057@gmail.com>
Commit-Queue: Albert Akmukhametov <alb.02057@gmail.com>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Reviewed-by: Никита Сказкоподателев (Nask) <skazkopodatelev@gmail.com>