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

Makefile

@@ -1203,6 +1203,9 @@ git-http-push$X: revision.o http.o http-push.o $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) \
 		$(LIBS) $(CURL_LIBCURL) $(EXPAT_LIBEXPAT)
 
+git-shell$X: compat/strlcpy.o abspath.o ctype.o exec_cmd.o quote.o strbuf.o usage.o wrapper.o shell.o
+	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^)
+
 $(LIB_OBJS) $(BUILTIN_OBJS): $(LIB_H)
 $(patsubst git-%$X,%.o,$(PROGRAMS)): $(LIB_H) $(wildcard */*.h)
 builtin-revert.o wt-status.o: wt-status.h