shallow repository: disable unsupported operations for now.

We currently do not support fetching/cloning from a shallow repository
nor pushing into one.  Make sure these are not attempted so that we
do not have to worry about corrupting repositories needlessly.

Signed-off-by: Junio C Hamano <junkio@cox.net>

receive-pack.c

@@ -421,6 +421,9 @@ int main(int argc, char **argv)
 	if (!enter_repo(dir, 0))
 		die("'%s': unable to chdir or not a git archive", dir);
 
+	if (is_repository_shallow())
+		die("attempt to push into a shallow repository");
+
 	setup_ident();
 	/* don't die if gecos is empty */
 	ignore_missing_committer_name();

upload-pack.c

@@ -672,7 +672,8 @@ int main(int argc, char **argv)
 
 	if (!enter_repo(dir, strict))
 		die("'%s': unable to chdir or not a git archive", dir);
-
+	if (is_repository_shallow())
+		die("attempt to fetch/clone from a shallow repository");
 	upload_pack();
 	return 0;
 }