In a bare repository, HEAD usually points to the branch that is
considered the "default" branch. As the current implementation for
`git_branch_is_checked_out` only does a comparison of HEAD with the
branch that is to be checked, it will say that the branch pointed to by
HEAD in such a bare repo is checked out.

Fix this by skipping the main repo's HEAD when it is bare.

Libraries should use assert(3P) only very scarcely. First, we usually
shouldn't cause the caller of our library to abort in case where the
assert fails. Second, if code is compiled with -DNDEBUG, then the assert
will not be included at all.

In our `git_branch_is_checked_out` function, we have an assert that
verifies that the given reference parameter is non-NULL and in fact a
branch. While the first check is fine, the second is not. E.g. when
compiled with -DNDEBUG, we'd proceed and treat the given reference as a
branch in all cases.

Fix the issue by instead treating a non-branch reference as not being
checked out. This is the obvious solution, as references other than
branches cannot be directly checked out.

We currently do not have any tests at all for the
`git_branch_is_checked_out` function. Add some basic ones.