Merge branch 'jk/printf-format'

Merge branch 'jk/printf-format'

Code clean-up to avoid using a variable string that compilers may
feel untrustable as printf-style format given to write_file()
helper function.

* jk/printf-format:
  commit.c: remove print_commit_list()
  avoid using sha1_to_hex output as printf format
  walker: let walker_say take arbitrary formats