sha1_name: warning ambiguous refs.

This makes sure that many commands that take refs on the command
line to honor core.warnambiguousrefs configuration.  Earlier,
the commands affected by this patch did not read the
configuration file.

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

tar-tree.c

@@ -380,6 +380,7 @@ int main(int argc, char **argv)
 	struct tree_desc tree;
 
 	setup_git_directory();
+	git_config(git_default_config);
 
 	switch (argc) {
 	case 3: