clone: simplifying dest repo creation
since creating a repo raises an exception if the repo already
exists, there's no need to try to create one.
add some test for repo creation when the destination exists
% file replaced with directory
adding a
% should fail - would corrupt dirstate
abort: file named 'a' already in dirstate
% directory replaced with file
adding a/a
% should fail - would corrupt dirstate
abort: directory named 'a' already in dirstate
% directory replaced with file
adding b/c/d
% should fail - would corrupt dirstate
abort: directory named 'b' already in dirstate