Link git-shell only to a subset of libgit.a

Link git-shell only to a subset of libgit.a

Commit 5b8e6f85 introduced stubs for three functions that make no sense
for git-shell. But those stubs defined libgit.a functions a second time
so that a linker can complain.

Now git-shell is only linked to a subset of libgit.a.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>