Merge branch 'jk/trace-fixup'

Various small fixups to the "GIT_TRACE" facility.

* jk/trace-fixup:
  trace: do not fall back to stderr
  write_or_die: drop write_or_whine_pipe()
  trace: disable key after write error
  trace: correct variable name in write() error message
  trace: cosmetic fixes for error messages
  trace: use warning() for printing trace errors
  trace: stop using write_or_whine_pipe()
  trace: handle NULL argument in trace_disable()

diff --cc cache.h
Simple merge