git-gui: use git-clone

git-gui clones a repository by invoking git-plumbing commands, in proc
do_clone, rather than using git-clone.  The justification was that the
low-level commands are guaranteed to provide a stable interface, while
the higher level commands such as git-clone may not be stable. This
approach requires git-gui to continually evolve by mirroring new
features in git itself, which has not happened, while the user interface
in git-clone has proven very stable. Also, git-gui does directly call
many other non-plumbing commands in git's repertoire.

do_clone's last significant functionality change was in 2015, and
updates are required for shallow clones, the reftable backend, cloning
from linked worktrees, and perhaps other features and bugs. For
instance, I had reports of git-gui failing to correctly clone
repositories prior to 2015, resulting in essentially the patch given
here. The only significant work was supporting .gitfile linked worktrees
unknown to do_clone, but supported by git-clone, and none regarding the
interface to git-clone itself. That interface is clearly stable enough
to not be a problem.

Supporting new use-cases with this requires exposing new options in the
clone dialog, then passing flags to git-clone. This avoids updating
do_clone to understand those options, reducing the maintenance burdens.

So, teach git-gui to use git-clone.  This change is in one patch as
there is no obvious incremental path to migration. The existing dialog /
options / status screen are unchanged, the known user-visible changes
are that cloning from a working directory linked by a gitfile now works,
there is no auto-fallback to a full copy when cloning linked workdirs
and worktrees (meaning git-clone fails unless a full or shared copy is
selected), and messages displayed are from git-clone.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>

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