Fix corrupted CloneCommand bare-repo fetch-refspec (#402031)

Fix corrupted CloneCommand bare-repo fetch-refspec (#402031)

CloneCommand has been creating fetch refspecs like this on bare clones:

[remote "origin"]
        url = ssh://example.com/my-repo.git
        fetch = +refs/heads/*:refs/heads//*

As you can see, the destination ref pattern has a superfluous slash.

It looks like this behaviour has always been the case for CloneCommand,
at least since cc2197ed when code catering to bare-clone fetch refspecs
was added. That was released with JGit v1.0 almost 2 years ago, so
there will probably be some bare repos in the wild which will have been
cloned with JGit and have these corrupted refspecs.

The effect of the corrupted fetch refspec is quite interesting. Up to
and including JGit 2.0, the corrupt refspec was tolerated and fetches
would work as intended with no indication to the user that anything was
amiss. With JGit 2.1, a change was introduced which made JGit less
tolerant, and fetches now attempt to update the non-existing ref
"refs/heads//master". No exception is raised, but the real ref -
"refs/heads/master" - is not updated.

This behaviour was noticed by a user of Agit (which does bare clones by
default and recently updated from JGit v2.0 to v2.2), reported here:

https://github.com/rtyley/agit/issues/92

If you run C-Git fetch on a bare-repo cloned by JGit, it flat-out
rejects the refspec (checked against v1.7.10.4):

fatal: Invalid refspec '+refs/heads/*:refs/heads//*'

Incidentally, C-Git does not create an explicit fetch refspec at all
when performing a bare clone - the full remote config generated by C-Git
looks like this:

[remote "origin"]
        url = ssh://example.com/my-repo.git

Using JGit on such a repository works fine, so omitting the fetch
refspec entirely is also an option.

Change-Id: I14b0d359dc69b8908f68e02cea7a756ac34bf881