branches: do not assert that the given ref is a branch

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.

src/branch.c

@@ -153,7 +153,10 @@ done:
 
 int git_branch_is_checked_out(const git_reference *branch)
 {
-	assert(branch && git_reference_is_branch(branch));
+	assert(branch);
+
+	if (!git_reference_is_branch(branch))
+		return 0;
 
 	return git_repository_foreach_head(git_reference_owner(branch),
 		branch_equals, (void *) branch) == 1;

tests/refs/branches/checkedout.c

@@ -37,3 +37,10 @@ void test_refs_branches_checkedout__worktree(void)
 
 	cleanup_fixture_worktree(&fixture);
 }
+
+void test_refs_branches_checkedout__head_is_not_checked_out(void)
+{
+	repo = cl_git_sandbox_init("testrepo");
+	assert_checked_out(repo, "HEAD", 0);
+	cl_git_sandbox_cleanup();
+}