remote: restrict default branch to branches namespace
travis: no need to clean out the test repository

This was added to avoid the remote's default branch to be considered to
the be notes one which the first network test leaves behind.
remote: assert what we want to happen when ther is no default branch

Assert what we already do, so as to notice changes.
remote: short-circuit the default branch check if there is none

If we do not have a HEAD ref in the heads, we already know there is no
default branch. Return immedately.
clone: support remotes with references but no branches

A repository can have any number of references which we're not
interested in such as notes or tags. For the default branch calculation
we only care about branches. Make the decision about the number of
branches rather than the number of refs in general.
remote: add tests for remote-branch edge cases

Add tests for the case when there are no branches on the remote and when
HEAD is detached but has the id of a non-branch. In both of these cases,
we should return ENOTFOUND.
remote: add test for single-branch clone

When cloning, we may be asking for a particular branch or subset of
branches. Make sure we test for that.
clone: correct handling of an unborn HEAD

If the remote does not advertise HEAD, then it is unborn and we cannot
checkout that branch. Handle it the same way as an empty repo.
clone: handle overly restrictive refspecs

When the fetch refspec does not include the remote's default branch, it
indicates an error in user expectations or programmer error. Error out
in that case.

This lets us get rid of the dummy refspec which can never work as its
zeroed out. In the cases where we did not find a default branch, we set
HEAD detached immediately, which lets us refactor the "normal" path,
removing `found_branch`.
Merge pull request #2511 from libgit2/cmn/remote-default-restrict

Restrict which refs can be the default branch