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