Check that parent_tag is in an ancestor of the checked out branch

`git rev-list A..B` returns the set of commits present in B but not
A. Only if A is an ancestor of B will the cardinality of the set be
the same as the forward-running offset from A to B.

TarSCM/scm/git.py

@@ -182,6 +182,30 @@ def _detect_version_tag_offset(self, parent_tag, versionformat):
             sys.exit("\033[31m@TAG_OFFSET@ cannot be expanded, "
                      "as no parent tag was discovered.\033[0m")
 
+        rcode, tgcommit = self.helpers.run_cmd(
+            ["git", "rev-parse", parent_tag + "^{}"],
+            self.clone_dir
+        )
+        if rcode:
+            sys.exit("\033[31m`git rev-parse " + parent_tag + "` failed: " +
+                     output + "\033[0m")
+        rcode, head = self.helpers.run_cmd(
+            ["git", "rev-parse", "HEAD"],
+            self.clone_dir
+        )
+        if rcode:
+            sys.exit("\033[31m`git rev-parse HEAD` failed: " +
+                     output + "\033[0m")
+        rcode, mgbase = self.helpers.run_cmd(
+            ["git", "merge-base", parent_tag, "HEAD"],
+            self.clone_dir
+        )
+        tgcommit = tgcommit.strip()
+        head = head.strip()
+        mgbase = mgbase.strip()
+        if mgbase != tgcommit:
+            sys.exit("\033[31mparent_tag is not an ancestor of HEAD. " +
+                     "Cannot compute a (meaningful) distance.\033[0m")
         rcode, output = self.helpers.run_cmd(
             ['git', 'rev-list', '--count', parent_tag + '..HEAD'],
             self.clone_dir