Merge branch 'xw/am-empty'

"git am" learns "--empty=(stop|drop|keep)" option to tweak what is
done to a piece of e-mail without a patch in it.

* xw/am-empty:
  am: support --allow-empty to record specific empty patches
  am: support --empty=<option> to handle empty patches
  doc: git-format-patch: describe the option --always

diff --cc t/t4150-am.sh
Simple merge

diff --cc wt-status.c
Simple merge