Allow explicit configuration of git directory in InitCommand

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