Merge branch 'cj/log-invert-grep'

"git log --invert-grep --grep=WIP" will show only commits that do
not have the string "WIP" in their messages.

* cj/log-invert-grep:
log: teach --invert-grep option

diff --cc Documentation/rev-list-options.txt
Simple merge

diff --cc contrib/completion/git-completion.bash
Simple merge

diff --cc revision.c
Simple merge

diff --cc revision.h
Simple merge