Use 'gitfile' in submodule checkouts

This change takes another step towards ensuring Git can understand
repo's submodules to some extent. Replace the old '.git' symlink with
gitfile[1] pointing to the bare checkout of the submodule. This is
required for Git's 'recurse submodules' opts to work with repo's
submodules as '.git' is expected to be writable by Git when recursing
over submodules.

[1] https://git-scm.com/docs/gitrepository-layout#_description

Change-Id: I52d15451768ee7bd6db289f4d2b3be5907370d42
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/446181


Tested-by: Kaushik Lingarkar <kaushikl@qti.qualcomm.com>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Nasser Grainawi <nasser.grainawi@oss.qualcomm.com>