This is similar to --with-linked-git, but:

1) It uses git submodule update --reference, so it works with submodules.

2) The created repo is a true git repo, except that its object database
reuses the referenced repo's objects, so it's a real speedup when e.g.
translations are enabled.

I intentionally didn't just fixed --with-linked-git, to make it clear
this is more like git clone --reference, not git-new-workdir.

Change-Id: I7c9584bce3670fd1e175b90aded2435cfe78056d
(cherry picked from commit 803137f4)

Conflicts:
	configure.ac