Yet more cleanup and fix test_file_id_f3 bogus test.

* bzrlib/tests/test_log.py:
(TestCaseWithoutPropsHandler.setUp,
TestCaseWithoutPropsHandler._cleanup): Delete gratuitous split
lines.
(TestShowLog.checkDelta): Give more explnations.
(TestShowLog.test_cur_revno): Simplified.
(TestShowLog.test_empty_branch, TestShowLog.test_empty_commit,
TestShowLog.test_simple_commit,
TestShowLog.test_commit_message_with_control_chars,
TestShowLog.test_commit_message_without_control_chars): Split
test_simple_log.
(TestShowLog.test_merges_nonsupporting_formatter,
TestShortLogFormatter.test_short_log_with_merges,
TestShortLogFormatter.test_short_log_single_merge_revision,
TestLineLogFormatter.test_line_log_single_merge_revision): Replace
try/finally clauses by addCleanup.
(TestGetRevisionsTouchingFileID.check_delta): Duplicate helper to
simplify tests.
(TestGetRevisionsTouchingFileID.test_tree_with_single_merge):
Simplified.
(TestGetRevisionsTouchingFileID.test_file_id_f3): Wow, align test
with comment, the real test was duplicated from test_file_id_f2
but only the comment was updated.
(TestReverseByDepth.test_shifted_merged_revisions): New test.