General code clean-up around a helper function to write a
single-liner to a file.

* jk/write-file:
  branch: use write_file_buf instead of write_file
  use write_file_buf where applicable
  write_file: add format attribute
  write_file: add pointer+len variant
  write_file: use xopen
  write_file: drop "gently" form
  branch: use non-gentle write_file for branch description
  am: ignore return value of write_file()
  config: fix bogus fd check when setting up default config