We _dispose_ the contents of objects; we _free_ objects (and their
contents).  Update `git_strarray_free` to be `git_strarray_dispose`.
`git_strarray_free` remains as a deprecated proxy function.

The repository URL is duplicated several times and can be de-duplicated
like this. Furthermore, exchange the static refspec variable with a
define to reduce BSS size.

There are only few actions which actually make sense for a detached
remote, like e.g. `git_remote_ls`, `git_remote_prune`. For all the other
actions, we have to report an error when the remote has no repository
attached to it. This commit does so and implements some tests.

The code used to rely on the clone code calling the remote's save, which
does not happen anymore, meaning that the configuration settings the
remote expected were not being written to disk.

The run-time configuration was still being affected, so the right branch
was being cloned. The tests continued to pass as we did not check for
the configuration entires. Fix this by creating the remote with the
single-branch refspec we want and checking for its existence in the
configuration.

Moved offending tests from network to online so they will get skipped
when there is a lack of network connectivity:
-test_online_remotes__single_branch
-test_online_remotes__restricted_refspecs