More simplifications.
* bzrlib/tests/blackbox/test_log.py:
(TestLogFile.test_log_file): Splitted into test_log_file1,
test_log_file2 and test_log_file3 with simpler coding.
(TestLogFile.test_log_file_historical_start,
TestLogFile.test_log_file_historical_missing,
TestLogFile.test_log_file_renamed): Simplified.
(TestLogFile.test_line_log_file): Deleted, covered by other tests.