Recent reimplementation of "git am" changed the format of state
files kept in $GIT_DIR/rebase-apply/ without meaning to do so,
primarily because write_file() API was cumbersome to use and it was
easy to mistakenly make text files with incomplete lines.  Update
write_file() interface to make it harder to misuse.

* jc/am-state-fix:
  write_file(): drop caller-supplied LF from calls to create a one-liner file
  write_file_v(): do not leave incomplete line at the end
  write_file(): drop "fatal" parameter
  builtin/am: make sure state files are text
  builtin/am: introduce write_state_*() helper functions