Allow cloning to an existing empty directory

The die() message updated accordingly.

The previous behaviour was to only allow cloning when the destination
directory doesn't exist.

[jc: added trivial tests]

Signed-off-by: Alexander Potashev <aspotashev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

dir.h

@@ -84,6 +84,8 @@ static inline int is_dot_or_dotdot(const char *name)
 		 (name[1] == '.' && name[2] == '\0')));
 }
 
+extern int is_empty_dir(const char *dir);
+
 extern void setup_standard_excludes(struct dir_struct *dir);
 extern int remove_dir_recursively(struct strbuf *path, int only_empty);