Report <2.5.1 in test_linked_worktree_traversal as error

Although GitPython does not require git >=2.5.1 in general, and
this does *not* change that, this makes the unavailability of git
2.5.1 or later an error in test_linked_worktree_traversal, where it
is needed to exercise that test, rather than skipping the test.

A few systems, such as CentOS 7, may have downstream patched
versions of git that remain safe to use yet are numbered <2.5.1
and do not have the necesary feature to run this test. But by now,
users of those systems likely anticipate that other software would
rely on the presence of features added in git 2.5.1, which was
released over 7 years ago. As such, I think it is more useful to
give an error for that test, so the test's inability to be run on
the system is clear, than to automatically skip the test, which is
likely to go unnoticed.

test/test_fun.py

@@ -279,7 +279,7 @@ def test_linked_worktree_traversal(self, rw_dir):
         """Check that we can identify a linked worktree based on a .git file"""
         git = Git(rw_dir)
         if git.version_info[:3] < (2, 5, 1):
-            raise SkipTest("worktree feature unsupported")
+            raise RuntimeError("worktree feature unsupported (test needs git 2.5.1 or later)")
 
         rw_master = self.rorepo.clone(join_path_native(rw_dir, "master_repo"))
         branch = rw_master.create_head("aaaaaaaa")