Merge branch 'rs/ref-fitler-used-atoms-value-fix'

"git branch --sort=..." and "git for-each-ref --format=... --sort=..."
did not work as expected with some atoms, which has been corrected.

* rs/ref-fitler-used-atoms-value-fix:
  ref-filter: remove ref_format_clear()
  ref-filter: move is-base tip to used_atom
  ref-filter: move ahead-behind bases into used_atom

diff --cc builtin/branch.c
Simple merge

diff --cc ref-filter.c
Simple merge