Merge branch 'hn/prep-tests-for-reftable' into jch

Merge branch 'hn/prep-tests-for-reftable' into jch

Preliminary clean-up of tests before the main reftable changes
hits the codebase.

* hn/prep-tests-for-reftable: (22 commits)
  t1415: set REFFILES for test specific to storage format
  t4202: mark bogus head hash test with REFFILES
  t7003: check reflog existence only for REFFILES
  t7900: stop checking for loose refs
  t1404: mark tests that muck with .git directly as REFFILES.
  t2017: mark --orphan/logAllRefUpdates=false test as REFFILES
  t1414: mark corruption test with REFFILES
  t1407: require REFFILES for for_each_reflog test
  test-lib: provide test prereq REFFILES
  t5304: use "reflog expire --all" to clear the reflog
  t5304: restyle: trim empty lines, drop ':' before >
  t7003: use rev-parse rather than FS inspection
  t5000: inspect HEAD using git-rev-parse
  t5000: reformat indentation to the latest fashion
  t1301: fix typo in error message
  t1413: use tar to save and restore entire .git directory
  t1401-symbolic-ref: avoid direct filesystem access
  t1401: use tar to snapshot and restore repo state
  t5601: read HEAD using rev-parse
  t9300: check ref existence using test-helper rather than a file system check
  ...