Allow explicit configuration of git directory in InitCommand

Native git's "init" command allows to specify the location of the .git
folder with the option "--separate-git-dir". This allows for example to
setup repositories with a non-standard layout. E.g. .git folder under
/repos/a.git and the worktree under /home/git/a. Both directories
contain pointers to the other side: /repos/a.git/config contains
core.worktree=/home/git/a . And /home/git/a/.git is a file containing
"gitdir: /repos/a.git". This commit adds that option to InitCommand.
This feature is needed to support the new submodule layout where the
.git folder of the submodules is under .git/modules/<submodule>.

Change-Id: I0208f643808bf8f28e2c979d6e33662607775f1f
Allow explicit configuration of git directory in CloneCommand

This feature is needed to support the new submodule layout where the
.git folder of the submodules is under .git/modules/<submodule>.

Change-Id: If5f13426cfd09b7677e23478e9700c8c25a6dae5
Support the new repository layout for submodules

When updating a submodule (e.g. during recursive clone) the repository
for the submodule should be located at <gitdir>/modules/<submodule-path>
whereas the working tree of the submodule should be located at
<working-tree>/<submodule-path> (<gitdir> and <working-tree> are
associated to the containing repository). Since CloneCommand has learned
about specifying a separate gitdir this is easy to implement in
SubmoduleUpdateCommand.

Change-Id: I9b56a3dfa50f97f6975c2bb7c97b36296f331b64