Merge branch 'rs/sha1-name-readdir-optim'

Optimize "what are the object names already taken in an alternate object database?" query that is used to derive the length of prefix an object name is uniquely abbreviated to. * rs/sha1-name-readdir-optim: sha1_file: guard against invalid loose subdirectory numbers sha1_file: let for_each_file_in_obj_subdir() handle subdir names p4205: add perf test script for pretty log formats sha1_name: cache readdir(3) results in find_short_object_filename()