Merge branch 'ps/ref-tests-update-more'

Merge branch 'ps/ref-tests-update-more'

Tests update.

* ps/ref-tests-update-more:
  t6301: write invalid object ID via `test-tool ref-store`
  t5551: stop writing packed-refs directly
  t5401: speed up creation of many branches
  t4013: simplify magic parsing and drop "failure"
  t3310: stop checking for reference existence via `test -f`
  t1417: make `reflog --updateref` tests backend agnostic
  t1410: use test-tool to create empty reflog
  t1401: stop treating FETCH_HEAD as real reference
  t1400: split up generic reflog tests from the reffile-specific ones
  t0410: mark tests to require the reffiles backend