Merge branch 'jk/pretty-empty-format'

"git log --pretty/format=" with an empty format string did not mean
the more obvious "No output whatsoever" but "Use default format",
which was counterintuitive.

* jk/pretty-empty-format:
  pretty: make empty userformats truly empty
  pretty: treat "--format=" as an empty userformat
  revision: drop useless string offset when parsing "--pretty"

diff --cc combine-diff.c
Simple merge