Make 2 more too-low git version skips into errors

In the tests only, and not in any way affecting the feature set or
requirements of GitPython itself.

This is similar to, and with the same reasoning as, cf5f1dc.

Author: EliahKagan

Diff for: test/test_repo.py

@@ -13,7 +13,7 @@
 import pickle
 import sys
 import tempfile
-from unittest import mock, SkipTest, skip
+from unittest import mock, skip
 
 import pytest
 
@@ -1235,7 +1235,7 @@ def test_merge_base(self):
     def test_is_ancestor(self):
         git = self.rorepo.git
         if git.version_info[:3] < (1, 8, 0):
-            raise SkipTest("git merge-base --is-ancestor feature unsupported")
+            raise RuntimeError("git merge-base --is-ancestor feature unsupported (test needs git 1.8.0 or later)")
 
         repo = self.rorepo
         c1 = "f6aa8d1"
@@ -1283,7 +1283,7 @@ def test_git_work_tree_dotgit(self, rw_dir):
         based on it."""
         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")