Use same code style for all logging without placeholders

When logging with only a msg argument, it is a full literal message
rather than a format string (as it is when there are placeholders).
Thus both `...("%s", text)` and `...(text)`, where `...` is a
logging method or function, are equally good code styles, provided
`text` really is known to behave the same as `str(text)`.

The latter style, `...(text)`, was used in all logging calls, both
in the git module and in the test suite, except one. This changes
the one outlier from `...("%s", text)` to `...(text)` for stylistic
consistency and to avoid giving the false impression that there is
something special about that call.

test/test_index.py

@@ -174,7 +174,7 @@ def _decode(stdout):
         except UnicodeDecodeError:
             pass
         except LookupError as error:
-            _logger.warning("%s", str(error))  # Message already says "Unknown encoding:".
+            _logger.warning(str(error))  # Message already says "Unknown encoding:".
 
         # Assume UTF-8. If invalid, substitute Unicode replacement characters.
         return stdout.decode("utf-8", errors="replace")