The code correctly detects that forced creation of a branch on a
nonbare repo should not be able to overwrite a branch which is
the HEAD reference.  But there's no reason to prevent this on
a bare repo, and in fact, git allows this.  I.e.,

   git branch -f master new_sha

works on a bare repo with HEAD set to master.  This change fixes
that problem, and updates tests so that, for this case, both the
bare and nonbare cases are checked for correct behavior.

refs: provide a more general error message for dwim

When we turned strict object creation validation on by default, we
forgot to inform the refs::create tests of this.  They, in fact,
believed that strict object creation was off by default.  As a result,
their cleanup function went and turned strict object creation off for
the remaining tests.

If we cannot dwim the input, set the error message to be explicit about
that. Otherwise we leave the error for the last failed lookup, which
can be rather unexpected as it mentions a remote when the user thought
they were trying to look up a branch.

When a non-empty directory exists and prevents the creation of a
reflog, provide a more informative error message.

When a (non-empty) directory exists at the reference target
location, complain with a more actionable error message.