Use a data factory for commit properties and reduce code duplication.

* bzrlib/tests/test_log.py:
(TestLogMixin.wt_commit): Helper acting as a data factory for
commit properties.
(TestCaseForLogFormatter): Use TestLogMixin and wt_commit()
(TestCaseForLogFormatter.assertFormatterResult): Get rid of
normalize_log() use.
(TestCaseForLogFormatter, TestShortLogFormatterWithMergeRevisions,
TestLongLogFormatter, TestLineLogFormatter, TestGetViewRevisions,
TestHistoryChange, TestLogWithBugs): Use wt_commit() and stop
using run_bzr().

* bzrlib/tests/blackbox/test_log.py:
(TestLog): Use test_log.TestLogMixin and get rid of
commit_options().
(TestLogMerges.make_branches_with_merges,
TestLogDiff.make_branch_with_diffs): Use wt_commit().