Merge branch 'ds/for-each-ref-is-base'

'git for-each-ref' learned a new "--format" atom to find the branch that the history leading to a given commit "%(is-base:<commit>)" is likely based on. * ds/for-each-ref-is-base: p1500: add is-base performance tests for-each-ref: add 'is-base' token commit: add gentle reference lookup method commit-reach: add get_branch_base_for_tip