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

contrib/completion/git-completion.bash

@@ -1425,7 +1425,7 @@ __git_log_gitk_options="
 # Options that go well for log and shortlog (not gitk)
 __git_log_shortlog_options="
 	--author= --committer= --grep=
-	--all-match
+	--all-match --invert-grep
 "
 
 __git_log_pretty_formats="oneline short medium full fuller email raw format:"