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

revision.h

@@ -169,6 +169,8 @@ struct rev_info {
 
 	/* Filter by commit log message */
 	struct grep_opt	grep_filter;
+	/* Negate the match of grep_filter */
+	int invert_grep;
 
 	/* Display history graph */
 	struct git_graph *graph;