Merge branch 'ab/strbuf-addftime-tzname-boolify'

strbuf_addftime() is further getting tweaked.

* ab/strbuf-addftime-tzname-boolify:
  strbuf: change an always NULL/"" strbuf_addftime() param to bool
  strbuf.h comment: discuss strbuf_addftime() arguments in order

diff --cc date.c
Simple merge

diff --cc strbuf.c
Simple merge

diff --cc strbuf.h
Simple merge