Merge branch 'gb/rebase-signoff'

"git rebase" learns "--signoff" option.

* gb/rebase-signoff:
rebase: pass --[no-]signoff option to git am
builtin/am: fold am_signoff() into am_append_signoff()
builtin/am: honor --signoff also when --rebasing

diff --cc Documentation/git-rebase.txt
Simple merge

diff --cc builtin/am.c
Simple merge

diff --cc git-rebase.sh
Simple merge