Merge branch 'jk/tests-timestamp-fix' into master

The test framework has been updated so that most tests will run with predictable (artificial) timestamps. * jk/tests-timestamp-fix: t9100: stop depending on commit timestamps test-lib: set deterministic default author/committer date t9100: explicitly unset GIT_COMMITTER_DATE t5539: make timestamp requirements more explicit t9700: loosen ident timezone regex t6000: use test_tick consistently