Fix TestBigReporR.setUp info message

Changes that fix the message itself:

- Add a missing space between words (two parts were concatenated,
  with no space at the edge of either).

- Capitalize "GitPython" since that is the repo and project name.

Changes that improve how the message is produced:

- Make the entire literal part of the string the format string,
  instead of formatting the first part and concatenating the second
  part.

- Pass the format string and k_env_git_repo variable as separate
  arguments to logging info, so the logging machinery takes care of
  substituting it for %s, rather than doing the substitution.

test/performance/lib.py

@@ -45,8 +45,9 @@ def setUp(self):
         repo_path = os.environ.get(k_env_git_repo)
         if repo_path is None:
             logging.info(
-                ("You can set the %s environment variable to a .git repository of" % k_env_git_repo)
-                + "your choice - defaulting to the gitpython repository"
+                "You can set the %s environment variable to a .git repository of your"
+                " choice - defaulting to the GitPython repository",
+                k_env_git_repo,
             )
             repo_path = osp.dirname(__file__)
         # end set some repo path